Determine the projects needs and purchase the Styrofoam needed from the local art store for small pieces or order from an online source for larger pieces (See reference 1).
Cut the Styrofoam using a Styrofoam cutter, bread knife or an electric knife.
Assemble the pieces together and use a hot glue gun to secure. Let dry.
Cover the surface with a material such as concrete patch mix. Concrete patch mix will seal the Styrofoam and create texture. Use disposable gloves and apply with a flat surface like a craft stick for small items and a hoe for larger items. Start by applying the mixture around one quarter inch thick and then adjust the amount for different types of thick or thin texture. Let dry.
Paint the project using a paintbrush, cloth or sponge.
Protect the surface from weather by applying an outdoor sealant or finishers.