Purchase plaster of Paris at a hobby or craft shop. Mix the plaster of Paris according to the instructions on the package. Typically, this will be one part water to two parts Plaster of Paris. Adding too much water will cause the plaster to crack.
Pour mixed plaster into a mold. Use a metal baking pan measuring 6-by-9 inches with a depth of at least 6 inches to give you room to bring out the relief effect of your design. You can use any type of mold, but this is ideal for a plaster relief carving. If you'd rather have a round carving, use a round baking pan as your mold. Use a knife or ruler to smooth the plaster flat, then let it dry for several hours (preferably overnight) before you begin working on your carving.
Remove your plaster from the mold. You may need to slide a knife around the edges to loosen it. Place your hand on the top surface and turn your mold upside down. Keep your hand in place as you shake the plaster piece from the mold. Place it on a sturdy, flat work surface for the carving process.
Draw your design onto the plaster with a pencil. If you can't draw by hand, there are plenty of stencil designs available at hobby and craft stores.
Use a flat chisel to remove the plaster from around the outside of your design. This will cause the design itself to rise from the background in relief. Cut low for a bas relief or dig deep into the plaster for a high relief design. To work on more intricate details on the design itself, you will need to use an X-ACTO knife with a small, sharp tip. You can also purchase a set of various size chisels at a hobby store.
Sand your design with a light-grit sandpaper until it has a smooth finish, then brush away the excess plaster dust before you apply paint. Use acrylic paints to color your relief design. Allow the paint to dry, then use a spray gloss coating to give it a shiny finish.