Warnings and Precautions:
If you are going to use someone else's blood, wear personal protective equipment and have proper documentation showing that they are STD and disease free.
Only use small amounts of blood to avoid serious injury.
Make sure you properly clean the area with rubbing alcohol or iodine, before and after you draw blood, to prevent infection.
Always use sterilized equipment.
Never cut deep. You could end up with a serious injury, infection, or scar.
Blood Magic: Introduction
Blood is a very powerful tool in magical practice, as it is viewed as a source of power and life force. It is a direct link to the individual from which it is taken and, therefore, it is believed that any act of magic worked with it will have a greater affect on the individual from which the blood was obtained.
Amulets, Talismans, Sigils:
Blood can be drawn and charged as a means to anoint amulets, talismans and sigils. The blood can be used to personalize the item in question, and attune its energies to that of the individual for which it is being created. This further strengthens the power of the item and its link to the individual that will be carrying or working with it.
Spells and Rituals:
In spells and rituals, blood can be used in much the same way we discussed above. Because of its power and its direct link to the individual from which it is obtained, it increases the power of spells and rituals worked on ourselves or other individuals. This works for both positive and negative acts of magic. I have personally found, that in this type of magic, the only away to cleanse or break the blood link is to burn the items on which the blood is attached.
It can be worked into poppets or smeared onto photos, and these items can be used as a focal point and fuel for spells and rituals. When you want to dismantle these spells and ritual items, burn them to sever the link.
Blood can be used as offerings to the spirits we work with and as a means to 'feed' thoughtforms, familiars, and other spirits.
If you have anemia I would highly discourage you from using blood simply because of the sheer amount of medical issues and concerns associated with the medicinal condition itself. You can use saliva in place of blood, in a case like that. It's still a link to yourself (or another individual) and biological material.
Blood use in rituals is a versatile practice. There really is no limit to what you can do with it. Most often, it is used (as mentioned above) for offerings, blood oaths with a spirit (I would not recommend this if you are newer to spirit work), as a physical/biological connector between the individual and the energy of the spell, etc. It can also be used, in combination with sex magic, as a means to symbolically connect two individuals on a physical and spiritual level. This is a ceremonial bonding of the energies of the two.
If you're seeking a specific example, I will use one where blood is used as an offering. Some deities and spirits prefer blood offerings, and in such cases it would be presented to them in much the same way that you would present them with herbs or whiskey. Call your quarters, cast your circle, perform your chants and prayers of invocation. Sit before your altar with your lancet or razor and meditate on the energy of the deity or spirit.
Place your hand, finger, etc over the offering bowl and draw blood. As you do so, give praise to your deity/spirit show thanks and gratitude as you offer them the blood. Blood is a very personal item to offer. It's a connection to you and thus it becomes a meeting point between you and the spirits or the divine when used in offerings or rituals centered around yourself.
In the same token, you can present the blood of your enemies to your spirits or deities. Ask them to 'feed' upon the blood of those that have wrong you and pass judgment or take action against the person upon your behalf.
Now, this is a very basic example.
Blood is just like any other spell or ingredient for ritual or a spell. You can use it as you see fit. However, I would not suggest using large amounts of blood for obvious reasons. If you are doing a ritual that involves blood drawing/bloodletting over a period of time to not take it from the same spot each time. This can lead to heavy scarring, slower incision healing, etc. I personally use razorblades and make small cuts on either side of my thumbs or fingers. A little can go a long way.