Sketch out your sculpture. Fill in the sizes and enough basic details to work out a rough model.
Choose some type of clay for a model. You can use a polymer clay, terracotta or anything else that will get solid.
Make a clay model of whatever you want to make. You can use your hands, clay tools or a knife to make it. Make a simplified, rough model with the same or slightly bigger dimensions as you want from your plaster sculpture. Let it dry or bake it as the clay requires. If you are using an earth clay, you do not need to fire it. Air-drying will work well enough.
Cover your clay piece with a thin layer of modeling clay. Then carefully add more and more modeling clay until it is surrounded by a thick mold of modeling clay.
Cut the modeling clay in half with a piece of wire, and carefully remove your clay sculpture. Carefully close the modeling clay mold again so that it is airtight, but has a hollow in the shape of your clay model.
Mix two parts plaster into one part water in a disposable cup. Stir it until it is smooth. Poke a small hole in your mold that goes down into the hollow, and pour in the plaster. Set it somewhere with the hole facing up, and leave the plaster to dry overnight.
Peel the mold apart or slice it open with a piece of wire. Use a craft knife such as an X-acto to carve the details of your sculpture.