1. From the moulding, measure both projection (P) and height (H).
2. Cut one board out of plywood, measured by the projection x 48". Mark the board as 1.
3. Cut two boards out of plywood, measured by the height + the thickness of the plywood x 48". Mark the boards as 2 and 3.
4. Pre-cut slots in board before assembling as shown in number 5. Place board 1 lying flat, then fasten board 2 and 3 vertically to the edge of board 1. Make sure all boards are cut and assembled "Square". Accuracy is important.
5. Cut two 45° (Clockwise) and -45° (Counter Clockwise) for the 90° corners. A 90° cut will allow you to do perpendicular cuts if needed.
6. Attach straps as needed to keep the mitre box, square and stable.