Back cuts Results in a closed end. Place the moulding upright in the mitre box. Make a 45° cut. Cut another moulding angled in the other direction, similar to cutting an outside corner. Mark the moulding at a 90° angle with a square and cut along the line. Assemble the moulding with the smaller piece and secure with glue and masking tape. The tape can be removed after 20 minutes.