Summarize:

Make enough to fill a container large enough to fit your entire hand. Choose a container that's also large enough so that your hand does not come into contact with the container itself. Be sure that the brand of rubber mold you use is non-toxic and safe for human contact. Hold your thumb and fingers in the pose you wish to replicate. Then dip your entire hand into the mold, keeping it well away from the sides and bottom. Leave at least a half-inch of mold between your hand and the container. Keep your hand posed within the mold for however long your mold's directions say it needs to set. Then, if needed, wiggle your thumb, fingers, and hand very gently to loosen them. Pull your hand out slowly. Avoid any quick, jerky movements that might disturb the cast you've just made. Prepare enough to fill the entire mold. Then pour a small amount (not all) of your plaster into the mold. Manipulate the container so that the plaster coats the insides of the finger- and thumb-holes in your mold. Allow the plaster to settle a bit to avoid the formation of air pockets. Fill the entire mold. Then bang the table a few times to release any air pockets. Quit once you no longer see any bubbles bursting on the surface. Wait for the time indicated by the directions to your plaster. Then upend the container and empty it onto the table. Make cuts into the rubber mold with a plastic knife and peel it from the plaster cast, piece by piece. Look for any imperfections where empty space in the rubber mold may have allowed room for extra plaster. Remove gently with sand paper or a small knife or file. Then either leave your finished cast as is or paint it as you desire.
Mix a batch of rubber mold. Soak your hand in warm water. Allow the mold to set. Mix a batch of plaster. Pour the rest of your plaster. Allow the plaster set. Clean the plaster cast.