First on your long wall put up your longest pieces. If you have to combine two boards, cut a 45 on the end of the two pieces, so that one overlaps the other. You should then be ready to cut the cornors. Turn your molding upside down on your saw. Cut a 45 on the end so that the angle is showing. Take a coping saw and follow the edge of the 45 were it starts at the face of the molding. Have a piece of sand paper or a round file handy. Make sure your molding is in equal measurements on the celing and wall.