Mix the prosthetic alginate with water,following the manufacturer's instructions, and apply it to the body part where you want to make a mold. Use the entire face if you are making a mask. Wear gloves and cover the area by hand with a relatively thin layer. Make sure to avoid trapping air bubbles under the alginate. If you are covering someone's face, do not cover their nostrils. The person will need to keep the eyes and mouth closed. Allow the alginate to completely set up.
Cover the alginate with a layer of plaster bandages to give it rigidity. Allow the plaster to completely set. As with the alginate, do not cover the nostrils, but completely cover as much of the alginate as possible. Once the plaster is dry, gently pull the alginate away from the body.
Melt the oil-based clay and pour it into the mold, covering the entire inner surface with a fairly thick layer of clay. Let the clay harden. You may want to do this several times to build up a thicker clay layer. Once the clay is hardened, gently remove it from the mold and set it face-up on a plastic sheet. Add more clay around the edges until there are no gaps between the clay and the plastic.
Sculpt any changes or corrections you wish to make in the clay for the finished mask or body mold. Carefully include all the details you will want in the finished product. Every detail will transfer into the finished latex.
Cover the entire clay model with plaster. Brush the plaster on gently until all of the clay is thinly covered, then apply enough so that the plaster is 1 to 2 inches thick on all parts of the clay. Allow the plaster to contact the plastic sheet to make a rim for the mold. Allow the plaster to completely dry, then remove the clay from it and turn it upside-down.
Pour liquid latex into the plaster mold, filling it completely. Let it sit for about 10 minutes, then pour the latex back out of the mold to leave a thin layer coating the mold. Allow this to dry in a well-ventilated area over night. Once it is dry, you can remove the latex from the mold and paint the finished mask or body mold however you choose.