Place the crown molding upside down into the miter box. The top of the molding should rest against the bottom of the miter box while the bottom of the molding rests against the side of the box.
Measure the wall where the crown molding will be installed. Hold the measuring tape on one end and have a partner take the measurement on the other end of the measuring tape. Write down the measurement and transfer it to the crown molding. Use this measurement to cut the crown molding to size in the miter box.
Determine the angle you need to cut on the crown molding. Most crown molding cuts are made at a 45 or 90 degree angle. Miter boxes are easy to use because they have slots that are easily marked at various angles. Insert the flat saw into the correct slot. Use the flat saw to cut the crown molding to size at the correct angle.