The Low-Down on Crown
Determine your backing situation. If you are using small (2-1/4 to 3 in.) crown, then the double top plate is probably reachable with finish nails. However, larger sizes of crown molding bridge between the wall and ceiling leaving a gap that is too large to be spanned by a finish nail.
For larger crown, install backer blocks. Set the table saw at 45 degrees and rip triangular shaped strips out of 2×4. Chop the strips to 6-in. lengths and install at 2 ft. intervals around the room, staying about a foot away from the corners. Mark the wall with painter’s tape a few inches below the blocks to indicate their position when you cover them with crown.
Measure from wall to wall for the first piece of crown.
Cut the first piece of molding square on both ends. Position the first piece with the detail down and the bottom edge at the top of the masking tape marks. Nail at the tape marks, which indicate the backing locations.
Masking tape on the miter saw table and fence marks the location of the crown molding edges so it is positioned at the correct angle to the saw blade. Since the crown goes in the saw upside-down, put the ceiling mark on the table and the wall mark on the fence.
Begin a cope cut by setting the saw at 45-degrees back toward the length of the board. Position the crown molding in the saw upside-down with the edges at the tape marks made earlier. Make your miter cut.
With the crown molding face down, hook the tape at the longest point of the 45-degree cut and mark the same edge of the molding at the wall-to-wall measurement.
Turn the molding face up and use a coping saw to cut along the line determined by the intersection of the face and the cut area.