Make a 45-degree cut
Inside corners are composed of a square-cut piece that butts the corner, and a corresponding piece that's cut on an angle and coped (carefully cut along the molding's profile) to conform to the first piece.
Begin by installing a piece of crown with a square-cut end (zero degrees on the saw).
Then position a second piece of molding upside down on the saw and adjust the blade for a miter cut. If this piece comes to the joint from the right side as shown in Step 3, swing the blade to 45 degrees left and make the cut. The bulk of the molding should be sitting on the left side of the saw.
If cutting a left-side coped piece, swing the blade to 45 degrees right, with the molding sitting to the right.
Cope the edge
Highlight the very edge of the cut, along the profiled front of the molding, with a pencil.
Next, using a coping saw, cut away the bevel. Angle the coping saw blade toward the rear about 5 degrees — a technique known as back beveling — and carefully follow the molding profile as you cut.
Use the miter saw table to brace the workpiece when cutting.
Check the fit
After completing the coped cut, test-fit it against the square-cut crown that's nailed in place.
If necessary, use a rasp to remove more wood or smooth out the coped cut.
Nail the coped molding in place and set the nails.
Cut the return
Where the crown molding doesn't butt ito something, you need to finish it with a return. Cut a 45-degree miter on the end of the crown (as you did for an outside corner) and nail it up. Next, make a 45-degree cut in the opposite direction on another piece.
Adjust the saw to zero degrees and lay the piece flat and face-down on the saw table. You'll need to remove your wood guide fence for this.
Cut through the crown, starting at the very point of the miter.
Complete the return
Bore two pilot holes through the tiny triangular return with a 4d finish nail chucked in your drill.
Apply a thin coat of glue to the return, then press it into place and hold it for a minute or two.
Nail the joint gingerly with 4d nails through the pilot holes only if the glue fails to hold.
Set the nails and fill all nail holes with putty.
