Things You'll Need
Styrofoam comes in many forms, some better for sanding than others. Most people are familiar with Styrofoam designed for packing. This is low-end Styrofoam, very large "grained" and not extremely strong. For craft projects, you should use foam core Styrofoam. Foam core is generally pink or gray and is much more ridged and dense than Styrofoam designed for packing material. With care, ridged foam core Styrofoam can be sanded to very exacting standards, making your finished product look very professional.
Mark lines to be cut with a straight edge and a pencil or sharp-tipped marker. Make cuts using a straight edge to ensure a straight line, always using an extremely sharp, new cutting blade. A thin-bladed knife, such as an X-Acto knife, often works best.
Wrap a sanding block with 80-grit paper or start with an 80-grit solid sanding block. Sand lightly, using very little pressure. At this stage you are only trying to sand down any high points and get the Styrofoam close to its final dimensions.
Continue sanding with a 120-grit paper. Again, sand gently, slowly bringing your Styrofoam into the final shape and size that you want. Use the 120 grit to smooth as well as to shape.
Finish your project with 200-grit paper. This should be used to smooth and "polish" your Styrofoam rather than for shaping. The last of your shaping should have been done earlier with the 120-grit paper. Once your Styrofoam is smooth it can be wiped off with a cloth and then painted with an acrylic craft paint if desired. A shiny acrylic craft paint can make your Styrofoam look like plastic.
One piece of Styrofoam can be used to sand another piece of Styrofoam.
Wear a breathing mask and eye protection if sanding with power tools.