In order to cut crown molding into angles, first place the molding in a miter box upside down with the molding against the sides and bottom of the box like it is against the ceiling and wall. Cut a 45-degree angle into the molding using a hand saw.
Cutting crown molding at an angle is necessary for inside and outside corners. For an inside corner, the top of the crown molding is shorter, while for an outside corner, the bottom is shorter. After cutting the angles, use a sanding block to sand the back of the cut. This allows the two corners to fit together tightly.
To install the crown molding, apply joint compound to the top and bottom of the molding, making sure not to use too much as this causes the molding to slide down the wall. The joint compound also needs to be applied to the cut sides of the molding. After the molding is on the wall, allow time for the joint compound to dry, then insert nails into the molding for support. Before attaching corner pieces, match up the profiles of the molding first. Then, adhere the molding with joint compound and use additional material to fill in any gaps.