Measure and mark how long the crown molding needs to be so you can cut it.
Place the crown molding on the miter saw. Set the molding on the saw so that the top of the molding rests against the table of the saw. The bottom of the molding should rest against the fence of the saw. The molding should look as it would look installed on the wall, only upside down.
Set the miter saw to a 45 degree angle. Hold the molding in place while you cut it at the 45 degree angle.
Mark the front, or leading edge, of the cut crown molding with a pencil. This will highlight where you need to cut with the coping saw.
Hold the coping saw blade toward the back of the molding at a 5 degree angle. Begin cutting away the back of the molding along the highlighted pencil line.
Place the crown molding in place on the wall to ensure that it fits into the other side of the molding. Nail in place with a pneumatic nail gun.