How to Build a Mirror From Case Molding

To size the plywood mounting board, add the casing widths to your mirror’s dimensions and subtract 1 inch from each side. The casings, resting flush on hardboard spacers, will overlap the mirror on their inside edges. Use a circular saw to cut the plywood and hardboard. Position the mirror and spacers on the plywood so that the spacers are flush with the plywood’s perimeter.