Note: This article will focus more on describing the practice rather than giving information on how the practice itself is actually done. This is because I do not recommend this practice to beginners, and do not feel comfortable giving out information on it that could be used without prior knowledge. As Gundarsson says in his work, to the unaccompanied beginner, soul-crafts are even more dangerous than more limited forms of magic such as galdr-craft. All sorts of magic can twist ones wyrd or cause harm unmeant; but when practicing soul-craft, you are travelling out into a perilous, unknown world filled with wights who may well not be friendly. On top of this, Soul-Crafts in general are very comprehensive and hard to cover briefly. Due to this, there may be some vague areas within the paper that I can address later on. Please read the sources section to see who credit belongs to for some of the research and information within the paper. And if terminology confuses you, please refer to the terminology section at the bottom.
What is Seidr?
I dont particularly want to debate the terminology of the word, as youll find its quite a headache. So skipping that Norse culture didnt actually have a specific shamanistic culture or defined practice that we know of. There was once a shamanic tribal culture in all of these areas. It was mostly gone well before the Christians converted everyone. Bits and pieces and glimpses of it can be seen and patchworked together, but we don't have a full picture of it from any written lore (Kaldera). However, the practice of Seidr bears strong resemblance to other shamanistic practices, and is therefore sometimes considered to be the Norse equivalent of shamanism. Please take note that this does not mean that sedir=shamanism, because it certainly does not encompass a lot of shamanstic features. Too often Seidr gets misused with a blanket term like witchcraft, which it was not. So as Gundarsson states: The chief problem in discussing seidr is defining precisely what seidr is.Neil Price, an archaeologist, summarizes Seidr as:  rituals for divination and clairvoyance; for seeking out the hidden, both in the secrets of the mind and in physical locations; for healing the sick; for bringing good luck; for controlling the weather; for calling game animals and fish. Importantly, it could also be used for the opposite of these things to curse an individual or an enterprise; to blight the land and make it barren; to induce illness; to tell false futures and thus to set their recipients on a road to disaster; to injure, maim and kill, in domestic disputes and especially in battle. This is what makes Seidr so complex and difficult to actually define: it is one practice with several practices within it. The Vitki and Volva (male/female practitioners respectively) performed many of the same practices that other shamanistic practices had already.
Looking into the surviving literature, we can paint of picture of what Seidr was like: If we examine the Ynglinga Sada, we are told of the magical arts of Odin. One specific part mentions: Odin understood the art that has the most power and used it himself, which is called Seidr (seething). And so he knew the fates of people and events yet to happen, could give death, ill-chance and disease to people; also he could steal wit and power from some people and give to others. And this sorcery, when it is practiced, had such an arge nature that the men thought it not without shame to deal with it, and so the art was taught to the Goddesses. Another part mentions Odin could change himself. His body then lay as if sleeping or dead, but he became a bird or a wild beast, a fish or a dragon, and journeyed in the twinkling of an eye to far-off lands, on his own errands or those of other men. Jan Fries examines the usage of arge, saying:  in modern German, means something like evil. In older times, the meaning was different had a variety of meanings, such as fearful, lustful, sinful, dangerous, which [gave] certain sexual qualities to the termIt is quite likely that the lustful tremors of the Seidr trance seemed shameful to the prudish men of Viking society.
If you examine many older Nordic tales, Seidr is often portrayed as a malicious magic. At times, terminology gets confusing at many seidr-workers called themselves spae-workers in order to avoid any negative connotations attached to the practice. Prophecy was one of the largest aspects of Seidr, which is why the line between Seidr and Spae-craft is blurred: seidr prophecy is more akin to spiritualism, mediumship and sorcery while spae-craft prophecy focuses largely on prophecy from ones own foresight or inner knowledge. The active compenent of seidr thus seems to chiefly be interfering with the soul-parts and/or consciousness of another notes Gundarsson, Snorri, of course- writing from a Christian perspective  - is not necessarily a reliable source in regards to specific details of magic. It could be argued that his dark portrayal of seidr was influenced by Christian hostility towards negative magic in general and towards that which might include the use of sexual energy in specific. We see Seidr noted as malicious several times, however, if we continue to look at the literature. A section of the Voluspa reads: She was called Hedr when she came to houses, a volva prophecying well/deceitfully. She knew gand-craft, she practiced seidr where she could, practice seidr, playing with hugr, she was ever dear to evil women. Hugr, in this usage, is a term that refers to a concept of soul controlling the psyche of a person. Again, we see this in the tale of Odin raping Rindr- as it is said Yggr performed Seidr at Rindr- leading us to conclude that he used Seidr to drive her temporarily insane, thus altering her state of mind.The Viking Lady says, The  most characteristic element of Seidr seems to be magic of a type which works by affecting the mind by illusion, madness, forgetfulness or other means, evidenced by the example above. Some sources, if you believe the credibility of them, mention that Seidr was used for magical attacks and in the literature it is seen that some seidr-workers used this practice as a direct means of bringing harm to others with it.
Seidr was said to include: sending nightmares and curses, love spells, causing restlessness and depression, manipulating the awareness of others through illusion, diving from the Gods and spirits, foreseeing the future, discerning fate, and so on. Positive uses of Seidr do occur in the literature, however. It is mentioned that a particular Volva uses seidr as a means of bringing fish back to a specific area, almost as though with a blessing and wish. A stone, as well, contains the line Thor, perform Seidr. Gundarsson notes, This strongly suggests that the skills of seidr could be used as warding techniques to good effectespecially if one is, as the Korpbron carver was, a Heathen living in a christian area (the inscription was put in coded runes inside a cross). Considering Thors usual means of dealing with threats, it does not necessarily suggest that his seidrhad good results for the person towards whom it was directed. It does, however, suggest that seidr was not always seen as a craft of ill; that at worst, like any sort of aggressive activity, the morality  depended on the circumstances under which it was deployed.
More modern practice of Seidr has become very reconstructionalist based, and has expanded to include more positive workings as well as more wide-based shamanistic practices, like healing, journeying or battling in animal form (the concept of altering your projected form/fylgja or animal guides, in a sense), projection methods (pathwalking and journeying, etc). It is important to note, however, that this is not what seidr was for traditionally and is not the primary function of seidr. Unlike other forms of shamanism, there was no concept of shaman sickness or initiation to begin practicing Seidr.
Unfortunately, there is STILL more to Seidr than that- however, I cannot go into great lengths about it at this time and don't wish to write another 5 pages. I'll pick this up and continue it later.
Gundarsson says, The practice of seidhr was particularly marked by three distinctive characteristics: the use of a platform or high-seat upon which the worker sat while performing his/her magic; the recitation of chants to bring the worker into a state of trance and/or to enlist the aid of outside wights; and activities which were considered to be ergi (sexually shameful), which, unfortunately for us, either were too disgusting for prudish sgumennto describe.
A volva was a woman shaman, a prophetess, a seeress, a priestess, and a magician in her own right. She divined with spirits and deities in order to learn the answers to questions through divination, provide those answers to the community, and otherwise work with spirits. She was supposed to look into the orlog (luck, karma) and wyrd (fate) of certain people, as well as work with the Norns to perceive and manipulate a persons wyrd/orlog and untangle any knots that were in them. Volvas practiced a variety of magick: Oracular Seidhr, Galdr, and Spae-craft.
The volvas were well respected. Throughout historical literature and mythological references, the volva is portrayed to be a very powerful influential woman. They were professionals, wise in their craft, in history, in the runes, in magick. The story of Volsupa, which means Volvas Prophecy, centers solely around Odin specifically consulting a (dead) volva in order to learn about his orlog, the future, wyrd, and such. Odin, the Allfather, actually gave this volva gold for her work- showing that even he respected her workings.
When they traveled into town, the people immediately flocked to her and allowed her to get to know the town/people and such. As an offering, people could kill a variety of nearby wildlife and beasts, then serve the volva the hearts of all of these animals. It was a form of sacrifice, in which the volva ate the hearts in order to understand the land better. The people of the town would set up a high seat (Seidhjallr)which had a cushion (often filled with feathers) placed atop it. Gundarsson suggests: The precise purpose of the high-seat is never detailed in the sources, but  it is the hallowed seat on which the sitter takes on all the might of his/her abilities, suspended between the earth and the heavens, the Middle-Garth and the Otherworlds, at the Well of Wyrd. If the reference in Eirks saga is not anomalous, sitting on this high-seat may also have made it easier for the seidr-worker to perceive the beings from whom s/he got lore and perform the function of a medium. Such a seat also makes it psychologically easier for the worker to detach his/her consciousness from the ordinary world and the folk around (if there are any), and enter into the state of trance required to work.
As the volva sat on the highseat, a chant was done to bring her into a state of trancework. There is some conflict in views on this, however. There was the vardlokkur, which is a song that uses magic and can bring a worker into a trance. However, the vardlokkur relies on galdr magic and is more of an invocation to spirits- so I would assume used more for spiritualistic type work, and divining from wight. This was sung directly by the person doing the work. However, another type of chant is mentioned: something akin to a wisdom chant. This was something recited, but rarely ever by the actual person doing the work. Often volvas and seidr-workers traveled in groups, therefore those watching the volva doing the actual work would then begin the chanting. After the altered state was reached, the main practitioner would go about their work.
It is important to note that in traditional society, Seidr was seen as an unmanly practice, and thus men who performed this type of magic were often looked down upon and called ergi and other derogatory names. Even in the lore, Loki pokes fun at Odin for practicing Seidr and calls him unmanly. It is theorized that seidr could have involved passive sexual reception or the use of sexual energies, even symbolically, such as the act of a wight riding the medium that is horsing them. Another thought is that the act of achieving trance requires a loss of self control, which was seen as unmanly in Norse society. Another theory involves that the seidr-worker had to look beyond the borders of genders, sometimes cross dressing, to achieve this high-self type of work. For whatever the reason, they all remain theories. Its not uncommon for men to practice it now.
What is Spae-Craft?
The Viking Lady defines spa as: the art of determining orlog (defined as the law of how things will be, laid down by wyrd or fate and the three Norns) usually by intuition or personal gnosis. Many of the Gods showed this ability:Odin and Freyja, for example. In the earlier days of our folk, the most honoured female leaders of the tribes were the spae-women who advised the war-chiefs concerning their battles with Rome. The most notable of these women was the Veleda, who fore-saw the victory of the Batavi and gave rede for the tribe to rise against the Romans in 69 C.E. (Gundarrson). This is not the only example within the literature, however. In fact, there are several instances where men and women possess fore-sight. Unlike seidr, the spae-craft was not seen in as a negative practiced and those who practiced it tended to be looked up highly, even honored.
How exactly spae-craft was practiced varies. Sometimes it seemed to call for special trance-techniques, as in the case of [Norse name] the Lawspeaker who went under the cloak for two days before deeming Icelands religious future  . Other times, as in [long Norse name of specific saga] spae-craft seems to have been a matter of psychic sensing with no special effort made. The matter was clearly a question of both personal character and the situation at hand (Gundarrson). Like the term volva in Seidr, another term for spae-workers comes about: Thule. Like the volva, the thule sits upon a raised seat and speaks inspiried words- believed to be from the Well of Wyrd. There, the thule has access to Oorlog and divines from this. The thule may have sat on top of burial mounds as well- giving in to the idea that the wisdom and information from the dead is given to, and channeled through, the speaker. This can overlap with the practice of mound-sitting. I do not particularly known much of spae-craft, nor practice it, so for now this is all I wish to add.
Dangers of Soul-Crafts:
Some of the hazards of this sort of travel include getting troll-shot (also: alf-shot, witch-shot, dwarf-shot),  having a part of your soul-complex stolen (in which case an experienced shaman has to be engaged to retrieve it) or eaten; being latched on to by an unpleasant wight which follows you about causing various sorts of trouble thereafter; and, worst of all, getting permanently lost  . (Gundarsson).
My personal advice is to not practice alone, if you can help it. It can be beneficial to have a partner who will watch over you while you are in an altered state doing this kind of work, and who knows enough about it that they could aid you if you had troubles. You can take precautions to help you as well: making protective wards, invoking a deity of your choice, taking protective talismans with you, etc.
Diana Paxson hosts a group called Hrafnar that consists of spae-craft workers/shamans. Workshops are sometimes held if you are interested in attending one. Learning the practice from someone with experience already is how I would recommend going about it: http://www.hrafnar.org/about-us/. This group is extremely skilled and knowledgeable in their work. If you are serious about learning, this is the place you will find a lot of research in the subject matter.
- Eddas- the primary sources of Norse lore
- Gothi/Gydia- a priest/ess.
- Seething- sometimes another word for Seidr, refers to seething body motions.
- Seidhjallr- the seidr platform, called the high seat.
- Seidkona- a female seidr worker.
- Seidman/Seidmadr- a male seidr worker.
- Vardlokkur- the song used in Seidr to achieve an altered state and invoke wights.
- Volva- another word for seidkona, a female seidr worker.
- Wight- another word for spirit.
- Wyrd- a concept of fate (http://www.spellsofmagic.com/coven_ritual.html?ritual=2217&coven=628)
- www.northernshamanism.org (Raven Kalderas site)
- www.hrafnar.org (credit to them for amazing research and study in the practice)
- Seidr: The Gate is Open by Katie Gerrard
- Seidways by Jan Fries
- Spae-craft, Seidr and Shamanism by Kveldulfr Gundarsson
- The Return of the Volva: Recovering the Practice of Seidr by Diana Paxson