So going under goes by a lot of different terms, depending on who you ask, what culture you're talking about, etc. Ill only be able to provide what I know, from my culture, so there could be differences if you asked someone else. Its known as Going Under the Cloak, Going Under the Hide, Going Under the Spirit, Sitting Out, etc. Basically, as the name implies, the idea is to be beneath something. Normally that something is going to be a cloak, hood or hide that will cover your entire body. Some serious practitioners follow after the Druids and place rocks up and down their chest to compress their breathing. The idea behind that is that the change in your breathing will serve as one of those altered states of consciousness, and make it easier for you to do your work and focus less on the external things going on around you. While some suggest that its more traditional for a three day sitting-out, they also advice most people NOT to do that. For one, if you've never been covered up for long- you may go a little batty the first time around. Not to mention the longer you stay under the more preparations you'll need to make.
Before even considering this, you need to make said preparations. I'd start by making sure you had a checkup with a doctor relatively close to the time you want to do this. That would ensure you don't have any nasty surprises while under. Another thing, it's not a smart idea to do this alone. Not saying you CANT do it alone, I've seen people who are fine by themselves, but it's generally safer and easier if you have a handful of trusted people to help you along. In most Norse workings you have a group anyway, so this part is not incredibly difficult. They play a certain role while you go under. For one, they are in charge of checking up on you without disrupting your work or distracting you from it. You would want someone in this group with some kind of medical experience, to ensure your physical safety if you are going under for a substantial period of time. From a magickal standpoint, they can do things such as set upwards or protection for you- to ensure your work goes as was planned, give you energy and otherwise help you along in subtle ways, etc.
One way of doing this in variation would be to dig a large hole in the ground, ideally out in the woods or some other peaceful environment. You don't want the sun shining directly in your face, so take into consideration your surroundings when you begin to dig. Once you've dug the hole, and you've got in it to make sure it fits you well, you need to consider how much of yourself you want to cover back up with the dirt. I find that if you cover all except for a small portion of your face (to breathe) the experience is a little more intense, and definitely not for everyone. The ground does have this weird feeling the longer you are in it. Its not necessarily bad. In fact, it often feels very warm and comforting. But it is a heavy-ish, closeness feeling and can cause a lot of issues if you are not good with tight spaces. Generally, depending on how long you are going to be in this hole, your body will have the occasional twitch or tremor, and you have to do your best to completely ignore it and otherwise not distract yourself. Trying to jerk around while you're buried isn't going to end well, so try calming yourself with breathing techniques. These techniques are also great for helping you achieve the desired state for your workings.
Now, if you're planning on doing this for a substantial amount of time there are things you do have to consider. For instance, I doubt you can hold your bladder forever. If you're under for less than a day, you could possibly try and hold it all in- providing you do not have any bladder, liver, kidney problems etc. Most people who go under for a long time such as this will fast in the days before, so there is less to actually have to prepare for or worry about. That would just leave the issue of said bladder. Depending on your personal preference, there's a ton of ways you can solve that: using a catheter, digging a deeper trench in a certain, specific place, etc. It's up to you to figure that one out. Another issue, occasionally, is the fact that you may become stressed for oxygen. If you're covering yourself all the way up, you are not getting the same amount as you usually would. I have known some people to take an oxygen tank with them for the longer durations. Water is another important factor. You're going to need it. You COULD have your companions occasionally pour water into your mouth for you, but I find that would be more distracting than you need. The smarter way to do it would be to get one of those camel-type bags with a long tube that you can easily rest in your mouth and get your own water without really having to focus on it.
If you are not comfortable with completely covering yourself in the earth, as the above technique describes, you can leave the majority of your body out of the dirt, and covered with a blanket, animal hide, or sheet, etc. You should probably get some mild-heavy stones and have your companions lay them over the edges of the blanket(etc) to keep it taunt and refrain it from moving around unnecessarily. This can be uncomfortable in other ways. If the blanket is dark or thick, you may notice yourself becoming a little paranoid or otherwise out of it. The lack of light, the combination of the heat from the ground and the moistness of your breathing (plus deprivation if you've been fasting) can lead to a lot of weird and sometimes frightening experiences. It may take you a while to get used to. A slight warning here though, in Northern tradition its said that it is completely possible to develop shaman sickness while you are under like that. That would include the Madness path or the spiritual and mental type of sickness that can develop while working under these conditions. So yeah, you might come out a few screws looser than when you went in. Keep that in mind.
If you have absolutely no desire to be in the earth whatsoever, there are other techniques (yay for you). Ideally, for this you want to go to a terrain you feel the most comfortable in. I personally like the woods, so I would go deep into the woods and find a suitable area that is grassy and comfortable to be in. Depending on preference, you may or may not wear clothing. (I tend to think you don't need it, but it depends on the climate of the area you're in, the likelihood of storms, etc). Once you've found your spot, you sit cross-legged and kind of hunched over. If you've brought a blanket or hood or hide you will drape this over your entire body (and again, it would be smart for your companions to place rocks on the outer limits of it to keep it down). If you don't want to use the blanket, cloth, etc, I've known shamans who take moss and leaves and other earthly things to cover their bodies. It's up to you, really. If you do take a blanket, it may come in handy. If it is hot outside when you are doing this, you can always douse the blanket in water and that will help regulate your body temperature. If it is cold, obviously, the blanket will help keep you warm. While hunched over in that position, you would begin to do your alteration of consciousness, via whatever technique you wish to use. Rhythm and breathing techniques are the most common here, though the plant usage could obviously have its benefits. When you come out of work, you may feel very sore and stretched. Its a combination of the way you were sitting and the duration of the whole thing, but otherwise isn't really an issue. Get one of your companions to give you a massage. That's why they are there.
If you don't like that one either (my you are a picky person..) there are things you can do with water. Float tanks are not uncommon to use, though they don't always achieve the same desired effects. For one, being in or a part of the earth feels much different from floating in water- and you may find it easier (in either or) to get into your working state depending on your comfort level in either situation. Helasdottir says, In contrast, float tanks are great, but that's very high comfort. When I do a float tank, I kind of feel like I'm cheating because it's not hard at all. It's really nice. It's dark and weird and creepy and you can't distinguish the boundary of your body from the water. Now obviously with this technique, you won't be able to do it for days on end like you would others, or at least I wouldn't recommend spending substantial amounts of time under the water. There's a lot of risk factors, such as falling asleep or otherwise becoming immobile while you are sitting out of your body. There's a lot of cramping and other feelings to distract you. You could have your crew help you a bit there, depending on the position in which you're laying in the water, but eh.
Sweat lodges are actually pretty traditional and a neat experience, though not for every person. Often saunas are considered a spiritual tool for Northern workers. I mentioned somewhere else that the sauna plays a role in deprivation techniques, and is used often for purification. Most workers tend to say the modern saunas, using electricity and whatnot, are not as spiritual nor appropriate for this working as a traditional one would be- though there may be more risk factors in the older workings. Here's what Raven K says you would need for a sauna:
A proper sauna/stofa ritual should have the following in attendance:
1) A source of wood heat, with real flame. Usually, this is a woodstove, although a stone hearth or oven will do just as well.
2) Ventilation. Many modern airtight saunas make people sick because the oxygen level falls too low. Even an open window to the cold is better than nothing - just crank the fire up. The ideal is an adjustable vent near the floor, to vent the cooling, sinking air, and another higher up to vent the excess heat later on.
3) Stones to throw water on. They can be collected ceremonially and charged with intent if you like. Do not use river stones, which have a tendency to blow apart during temperature changes. Water to throw on the stones, preferably rainwater.
4) A birch whisk. To make this, collect birch "twigs" - meaning branches less than two feet long - and tie them together. It is best made and used fresh, but of course, you may not be able to get fresh leafy birch twigs for a good portion of the year. Think ahead and make a bunch of them, and let them dry. You will use each one up every time you do a sauna ritual, so be prepared. (If you have a chest freezer, you can freeze them flat in bags and then thaw them later.)5) Knowledge of the proper sauna etiquette. A sauna is not for partying, rowdiness, or fondling each other. It is a solemn occasion, and a quiet, meditative ambiance should be promoted. Being naked is mandatory; one should go in as one came out of the womb. The sauna is a rebirth experience in its own way. In our modern society, some people may feel shy about being naked, but this is fairly critical. Anyone who would be so rude as to comment on someone's body, or give someone an unwanted touch, shouldn't be allowed to be present during such a ritual anyway.
There's a ton more that you can do. You can cocoon yourself in a lot of blankets, or even a mummy bag. You can use earplugs, blindfolds and other things to help deprive you of your senses to make other techniques more beneficial to you. Getting a body bag, using a blindfold, earplugs and maybe even a gag, and just laying in the darkness with no sound, etc, can actually be a really simple way to do this. Anyway. If you plan on doing something like this, definitely spend time preparing yourself and thinking over the whole situation. You can do these in combination with certain things like ordeal work, deprivation work, awareness techniques, consciousness techniques, etc- if you're wanting something more powerful. Most who do this go in with the desire to commune with the wights, or with a specific deity, and do projecting while under these states (hence sitting out). However, its probably a very bad idea to try and channel a spirit or deity while in those conditions. I don't think Ive met any practitioner who has attempted it in those situations. I imagine it would be very labor-intensive and difficult. Probably confusing as well. Seidhr and other workings similar to it can be done during this time, though it may take a bit of getting used to working with it under these new conditions and etc. It makes it much easier for your companions to watch your body when you're setting out, considering in most of these situations you are immobile. Divination and healings are also very common to do when working on this angle.
Some more sources:
Wightridden: Paths of Northern-Tradition Shamanism: Raven K, Lydia Helas.
Odin: Miercinga Rice
Anna Franklins works
Gisela Ko's works
Warding: Lorrie Wood