Find a cardboard box large enough to hold your body part to be molded. Tape up the bottom and sides. Cut a notch on one side of the box for use later. Mix your plaster in a large bowl or bucket according to the package directions. Pour the plaster in the box three or four inches deep. Cover the body part in petroleum jelly, which is very important to avoid burns.
Place the body part (hand, foot, leg) to be molded into the box and gently move it side to side to get it all the way in but make sure it doesn't go all the way to the bottom of the box. Let the body part sit in the mold for 15-20 minutes while the plaster hardens. The plaster mold will become very warm, don't worry this is normal.
Leave the body part in the mold and add petroleum jelly to the top half of the body part and the mold. Pour more plaster in the box so that it covers the body part entirely. Allow the plaster to sit for 15-20 minutes. Then carefully remove the top layer of the mold. Let the body part model remove their body part. Let both molds continue to sit out for an hour.
Clean out the molds to remove any debris. Then cover both halves of the mold in petroleum jelly. Apply a think layer of chalk on the outside of one half of the mold for removing later. Put the molds back together in the box. Duct tape the molds tight. Tilt the box sideways so the notched end of the box is facing upward.
Pour ballistics gel into the notch so it fills the mold. Place the mold in the fridge for several hours. Take the mold out of the fridge and carefully pull the body part out of the mold. The petroleum jelly should make it slide out smoothly. Wipe off any excess petroleum jelly and dry it off. The final step is to paint the body part with latex paint to make it look life-like. That's it, have fun!