Decide what type of sculpture you want to create, and purchase appropriate Styrofoam pieces from a local art store. Styrofoam pieces are usually located in the floral section, and they come in a variety of sizes and shapes including balls, triangles and rectangles.
Use a bread knife to cut Styrofoam if needed. Assemble pieces with a hot glue gun, and then set them aside to dry.
Use a concrete patch mix, which can be found in home improvement stores, to create a concrete effect. Apply the concrete with craft sticks or disposable gloves, and make sure they are approximately 1/4 inch thick.
Use a sponge or paintbrush to paint the sculpture if desired. Embellish with decorations such as beads or stones. Use the patch concrete to create molded accents, or use stencils to create accent pieces using concrete or paint. You can use stamps to imprint letters and designs directly into the Styrofoam or concrete patch.
Apply a sealer or finisher for outdoor protection.