Watch this video from This Old House to learn how to make an inside joint for crown molding.
1 Make a miter-cut on the end of a length of molding.
2 Take a pencil and darken the leading edge of the mitered end.
3 Cut along darkened edge with a coping saw, angling the blade back as you follow the curved profile of the molding.
4 Smooth out the rough edges of the coped cut with sandpaper or a round file.
5 Install a square-cut length of molding first, then butt the coped-cut piece into it to form a tight-fitting inside corner joint