Watch this video from This Old House to learn how to cut crown molding.
1. To make a scarf joint in the middle of a long run, start by placing the crown molding upside down on miter saw table; hold the molding at the same angle as it will be installed.
2. Set blade at 45 degrees and make cut in two lengths of molding to create a scarf joint.
3. To form an outside corner joint, set the saw blade at 45 degrees to the left; cut the first length of molding.
4. Rotate saw to 45 degrees to the right and cut the second mating molding piece.
5. For an inside corner joint, square-cut the first piece of molding and butt it tight into corner.
6. Miter-cut the second piece to 45 degrees.
7. Highlight the leading edge of the molding with a pencil.
8. Cut along leading edge with a coping saw.
9. Smooth the cut edge with 100-grit sandpaper to create a tight joint.