CUT AND SHAPE the back panel good side up. If you use the measuring technique shown on step 3, place masking tape where the center line will be and mark it so you can accurately transfer your measurements. Use an ink pen (tape optional) to mark the cutting lines. To remove 1/2 in. or more of material, use a circular saw equipped with a masonry blade. Smooth the circular saw cuts and make smaller adjustments (like tapers) with a belt sander and a 100- or 120- grit belt.