Create a design for your sculpture. Draw or find several views of the sculpture's subject. It is best to have at least one profile view and one front view of the sculpture drawn or printed out on paper. A top view is also useful. Shrink or enlarge your drawing using a photocopier until it is approximately to scale at 1 inch on the drawing to 1 foot on the sculpture.
Draw a grid of 1-inch squares across the face of the drawing or printed image. This will aid you in transferring your design to the styrofoam.
Purchase a block of styrofoam large enough to cut your sculpture from. Bulk styrofoam sellers can be found in most medium-to-large cities. The foam is typically sold by the pound. Large pieces may need to be ordered, or smaller pieces can be glued together using water-based styrofoam adhesive caulk, available in hardware stores.
Draw a grid of 1-foot squares on the foam with a permanent marker. Transfer the lines from the squares of the 1-inch grid on your paper to the grid on your styrofoam. Do one square at a time, beginning in one lower corner. Next, draw the adjoining squares and continue until the entire design has been transferred. Draw at least one side view, or profile, and the corresponding front view onto the foam.
Cut out the basic shape of the side view with your cutting tool of choice. Electric chainsaws, electric cutting knives, reciprocating saws and hand saws all work well. A large hot-wire cutter is the fastest cutter and will leave the cleanest line. Make sure you have plenty of ventilation and use a dust mask to avoid inhaling the dust.
Carve the corners and other facets of the front view into your styrofoam. Shift to finer tools as the detail of your cuts increases. An electric carving knife works best in this stage.
Cut the back view of your sculpture in the same way. Take time to line your cuts up with the profile and front views to create a smooth three-dimensional shape. Continue cutting away until the block of foam has the shape you want.
Carve details using small, fine tools such as rotary craft tools, utility knives, wood rasps and grater planes. Almost any tool with a cutting surface can be used. Rotary craft tools with fine-grit cutting stones work best for smoothing out areas that need a smooth texture, such as areas representing skin.
Apply a mixture of one part white glue to two parts latex semigloss paint to the surface of the sculpture to seal it. Apply two coats, allowing it to dry completely between coats. Latex paint or other coatings can be added on top of this sealer if desired.