Step 1
Unscrew the hinge from the door to remove the old cabinet door. Save the screws and set aside.
Step 2
Set the old door over the plywood sheet and trace the outline or transfer the measurements with a tape measure to the new board. Cut out a new board with the same dimensions as the old board, which will be 1/2-inch by 24-inch by 24-inch.
Step 3
Position four 1/2-inch by 3-inch by 21-inch boards with largest sides down in the shape of a square frame, which should be the same size as the plywood door board. Set a mirror which is four inches smaller in length and width on top of the square frame with the mirrored surface facing down. Arrange this 3/32-inch by 20-inch by 20-inch mirror evenly on top of the square frame so there is an equal amount of space all around.
Step 4
Position the plywood board evenly on top of the mirror and frame. Drill holes every 6 inches through the plywood board and into the frame, being very careful not to damage the mirror or go through the frame boards. Set 3/4-inch wood screws in each hole and tighten completely to secure the mirror in place.
Step 5
Position the new door against the cabinet opening and line it up with the hinges. Mark the locations of each of the hinge screw holes on the back of the new door board. Drill a hole into each marking, being careful not to go all the way through the frame. Set 3/4-inch screws in each hole in the hinge and line them up with the holes in the door. Tighten the screws to attach the doors to the hinges.