Liberally apply a layer of petroleum jelly to the body part to be molded. The jelly will protect body hairs. Layer it on thickly or the mold will be painful to remove.
Mix the alginate with water with a whisk according to the manufacturer’s instructions until it becomes semi-fluid. Alginate is a rubbery material that captures the fine details of a body part and forms the basis of your mold.
Spread the alginate onto the body part, applying the material in several layers. Wait three to eight minutes for it to set, depending on manufacturer's instructions.
Place three layers of plaster bandages over the alginate and wait for them to become firm, generally between between three and six minutes.
Carefully cut a slit through the mold using the scissors. Safety scissors will prevent you from cutting the body part.
Slowly remove the alginate. Ask your model to wiggle the body part slightly to aid in the removal.
Quickly apply strips of plaster bandages to the slit in the mold, closing it before the alginate begins to deform at the slit.
Fill the mold with plaster. For best results, use a high grade plaster intended for model creation, found at art supply stores. Allow the plaster to dry according to the manufacturer’s instructions.
Remove the plaster bandages and the alginate, revealing the cast body part. If some pieces are difficult to remove, carefully cut them away using a razor knife.