To cut crown molding corners, first put the molding upside down in a miter box, making sure it is braced on the back and side of the box, and then cut the molding at a 45-degree angle with a hand saw. This method works for inside and outside corners, but the top part should be shorter for inside corners and the bottom part should be shorter for outside corners.
Before installing the cut crown molding in the corners, shave down the back of the molding pieces so there is room for the two corner pieces to fit tightly together. To install the crown molding, use the right amount of joint compound to stick the molding to the wall without it sliding down. Apply the joint compound to the top, bottom and cuts of each piece of crown molding with a putty knife. Put the two inside corner pieces together, and then fill in any holes or gaps with more joint compound. It is best to try to fit the corner pieces together before applying them to the wall to make sure they match up properly.
When applying outside corner pieces to the wall, push the two pieces together until the joint compound comes out of the joint. For best results, remove excess joint compound from the walls and ceiling while it is still wet.