To cut a 90-degree inside corner in crown molding, use a coping or miter saw to cut one piece at a 90-degree angle and another at a 45-degree angle with the lower edge longer than the upper. Cutting the upper edge longer than the lower results in an outside corner.
The use of a coping saw creates a tighter joint and allows for the use of caulking to hide any small imperfections in the joint. A power miter saw eliminates the need for a miter box to guide the angled cut and creates a cleaner cut.
Measure the length of the trim piece that you need, and then mark and cut the first piece of crown molding to fit flush against the wall. Measure and mark the second piece of crown molding. After verifying that the edge that sits lowest on the wall after installation is longer, cut the trim piece at a 45-degree angle using a miter box and coping saw or a power miter saw adjusted to a 45-degree angle. Place the second piece of crown molding against another piece at a 90-degree angle to check the cut. Use a coping saw to carefully remove any high points in the edge that prevent the creation of a tight joint.