COPING INSIDE CORNERS
A quick summary: Cut the first piece at a 90° angle, butt it into the corner, and nail. For the piece that will be coped, use a board longer than needed, since you will cut it to length after making the joint. Cut the second piece (using a miter saw or miter box and hand saw) at a 45° bevel, with the back side longer than the face. After you cut away the back portion using a coping saw with a fine-tooth blade, this profile will follow the contours of the piece already on the wall.