Crown moldings can add that finishing touch to a room -- except when the corners don't fit together. Simply cementing together two mitered pieces of molding into a corner looks unprofessional, especially when the inside corners of a room aren't angled at exactly 90 degrees. The solution is to make a coped joint.
You'll need a basic coping saw with not too many teeth. This will make it easier to cut through the relatively thick crown moldings. Many carpenters prefer to cut on the pull stroke (the teeth of the blade facing the handle), while others find it easier to cut on the push stroke (blade teeth facing away from the handle). Choose whichever way is most comfortable for you. It's best to practice first with a small, spare piece of molding to determine the best angle to cut at.
First, cut the piece of molding so that it can sit flush against the wall. Next, clamp the molding upside down inside your miter box. Use a stop on your extension table to hold the crown molding firmly at the desired angle. Cut the profile of one end of the molding at a 45-degree angle so that you are sawing off more material from the back side of the molding. Larger crowns will be trickier to cut, as the angle of the cut needs to be approximately 50 degrees. Depending on the profile of your molding, you may need to start with a square cut and only later begin to angle the cut. If your saw blade begins to divert from your cutting line, pull the saw out and cut from the opposite direction. Set the cope against another piece of molding to check how well they sit together. File down the edge with sandpaper until you get a perfect fit.