Spread a plastic drop cloth over your entire work area. Wear rubber gloves and an apron during the plaster casting process.
Determine the volume of the inside of your mold. You can fill the mold with water and pour it into a graduated pitcher to find out what the volume is. Pour this amount of water into a plastic container at least twice as large as the volume of water.
Read the label of your dry plaster to find out how much plaster to add to the amount of water you have. In general, you will probably need two parts of plaster to one part water. Do not mix the plaster with water yet.
Clean and dry your mold. Coat the inside of your mold completely and evenly with a releasing agent. Petroleum jelly and cooking spray are good releasing agents. If the releasing agent does not cover all parts of the inside of the mold, including small crevices, the hardened plaster will not come out.
Build a framework around your mold if it is flexible and would buckle under the weight of wet plaster. Silicon or latex molds usually need this superstructure. Build a framework out of Foamcore, heavy cardboard or even plywood, if necessary, that supports the mold. Hold Foamcore or cardboard framing together with hot glue or wire. Use wire or nails for a plywood frame.
Pour the dry plaster into the water when you are ready to cast it. Wet plaster has a working time of only a few minutes, so wait until your mold is ready before you mix the plaster. You will have only about 15 to 30 minutes to work once it is mixed. Stir constantly using a wooden stick or plastic spoon as you pour the powder into the water. Do not mix the plaster with your hands. Stir until the consistency is even throughout and thin enough to pour.
Fill the prepared mold half full with wet plaster. Shake the mold gently and tap the sides of it to dislodge air bubbles. If your mold is very large or if it has broad surfaces that are very thin, insert metal wire or rods into the plaster while it is wet to give reinforcement to the casting. This is similar to placing rebar in concrete.
Fill the mold to the top with wet plaster. Shake gently and tap to get out all of the air so there are no pockets. Let the mold sit undisturbed for several hours until the plaster hardens. This can take up to 24 hours.
Clean your implements immediately and thoroughly after mixing and casting. For small disposable implements and containers, allow the plaster to dry and throw them in the trash. Never wash wet or unset plaster down the drain. It can harden in the plumbing pipes. If you have larger utensils you want to reuse, clean them in a bucket of water, not the sink. Wipe them off with paper towels. Let the plaster sediment sink to the bottom of the bucket and carefully pour off the water on top. Let the sediment dry, flex the plastic bucket, and throw away the plaster pieces that fall off. Allow the paper towels to dry and throw them away.
Remove the mold from the plaster. If the mold is flexible, gently twist it to loosen, then pull it away from the casting. If it is a rigid mold, gently pull the mold parts away from the plaster inside. If the casting feels damp, leave it alone until it is dry and hard.
Sand the dry casting lightly to smooth out the surface. Wear a dust mask to protect your lungs. You can carve and sculpt the dried plaster casting if desired.