Get a large enough bucket to fully submerge the body part you want to make a mold of. You should be able to stick the part in without touching the sides or bottom.
Add water and mix in alginate with a hand blender or whisk until there are no lumps. The ratio is usually about 1 to 1, but you should follow the directions on the package of alginate.
Stick the body part into the mixture and hold it there for 8 to 10 minutes, keeping it as still as possible.
Slowly and carefully pull the body part out. If you go too fast, you can tear the mold and ruin it.
Spray spray-on cooking oil into the mold. This step will help you slide the cast back out of the mold once it has set.
Mix enough plaster of Paris and water to fill the mold. Mix 2 parts plaster with 1 part water and pour it into the mold.
Wait overnight for the body part to set. Peel the mold away from the plaster. You now have a cast of a body part.