Step 7: Cope Inside Corners
The right side will be butted and the left will be a coped miter. To make the coped joint, miter the left end of the molding and then back cut it with a coping saw.
Be sure to follow the trims profile as a reference for the blade. Then, file the edges clean, so the profile slips neatly over the previous piece; creating an inside corner.
Fasten the piece and continue the process of measuring, cutting and fitting as you work your way around the room.