To get a tight seam for your crown moulding, cut a cope joint instead of using a mitre cut
In theory, you could miter crown moulding when two pieces meet in a corner. However, that would be a disaster. Crown only touches the wall at two points, the top and the bottom, so the vast majority of the joint is floating in midair making it impossible to get a tight seam. Coping the corner pieces solves the problem. Here's how it works: One moulding runs into the corner of the room, where it meets the wall with a simple butt joint. It supports the second piece, the end of which is cut to fit against the first like a piece of a jigsaw puzzle.
Putting the first moulding in place is a matter of cutting a butt joint and nailing the moulding in place. Cutting the second moulding to fit against it begins with a miter cut that makes it easy to see the shape you need to cut. You then cut along the shape with an inexpensive handsaw called a coping saw. Make sure you have got a fine blade in it. You will also need a stable work surface one to three feet above the ground--a toolbox or sawhorses will do, but a platform supports more of the moulding and is easier on your back.
This guide highlights how to cut the end of one moulding with a mitre saw and then cut the cope joint.
What You'll Need
Nail the first piece of moulding in place. Hold the second piece temporarily in place, and lay out the direction of the miter by drawing a line that starts at the bottom of the moulding and angles up and away from the corner. The line is only a guide and doesn't need to be drawn at any particular angle. Put the moulding on the miter saw and set the saw to cut a 45-degree miter that angles in the general direction of the line you drew and make the cut.
Put the saw blade on the end grain just below the surface of the moulding, as shown. If the moulding is primed, you will lay the saw just a hair below the lower surface of the paint. Make a few passes with the saw, making a shallow groove along the entire length of the flat.
Once you have made a shallow cut into the flat, lift the saw handle and begin cutting along the bottom corner of the moulding. Cut from the corner to the point where the cove begins, following the groove you cut in step two.
Back the saw out of the cut you have made. Start a new cut from the back of the moulding. Cut up towards the middle of the cove and then follow the shape of the cove until you reach the cut you finished in step three. Remove the waste.
Turn the saw around and cut along the profile of the upper section of the cove. Stop cutting when you reach the fillet.
Come in from the back of the moulding and cut along the fillet until you reach the cut you made at the top of the cove.
Start at the fillet and cut along the S-curve until you reach the flat at the top of the moulding.
Put the saw on the end grain just below the surface of the moulding. Make a few shallow passes and then raise the handle so that the saw is cutting deepest at what will be the top edge of the moulding. Continue cutting until the saw meets the cut along the S-curve and the waste falls away.