In order to neatly install crown molding at angles along wall corners, either cut a cope at the corner pieces for a tight fit or install corner blocks to hide the gaps. Dry fit all of the trim pieces before nailing them in place, and finish the molding job by caulking all of the joints to fill in any gaps.
The problem that installers often find when installing crown molding is that the walls and corners may not be fully square, which can leave uneven lines with large gaps. The large crown molding pieces are cut at a 45-degree angle to connect to one another, but with an uneven corner, the molding will not properly line up. Caulking the finished product can reduce these gaps, but a proper installation will go farther than caulking can.
Installers have a few options for neat trim in the corners, including coping the molding or installing a corner block. Coping corners allows installers to back-cut the corner pieces so they fit tightly against the walls and other trim pieces. To cope the molding, use a coping saw and cut away the material behind the trim face. If coping is not practical or possible, installers can install corner pieces in the interior corners of walls so they can hide any gaps or uneven trim pieces.