Keep It Cold
The first thing to do with a blood stained garment, whether recently stained or not is to wet it thoroughly with cold water (under a tap works best). This loosens up the proteins in the stain making it easier to clean.
Note: Hot water has the adverse effect, making stains more difficult to remove.
After a few minutes of the garment soaking, add up to a cup of salt to a bucket or mixing bowl of cold water. Add the garment and let it sit for 30 minutes.
Soap It Up
After the 30 minutes have passed, remove the clothing from the bucket and using a bit of soap on a cloth, wipe the stained area. By now the stain should be significantly gone.
The final step in the process is to add a bit of hydrogen peroxide to the stained area. Be sure to use a 3% concentration (the type sold for first aid) as it is not strong enough to bleach clothing. Let the garment sit for a couple of minutes and rinse thoroughly with cold water while rubbing the stain.
The stain should now be completely removed. If not, repeat Steps 2-5 as needed.