Measure the door gap in the cabinet, and add ? inch to each side.
Saw off the ?-inch plywood into the four previously measured sides using a miter square. Use two pieces for the horizontal frame sides, called "rails," and the other two for the vertical frame sides, called "stiles." Use the router to make a coping cut on both ends of each rail and grooves on both ends of each stile. Attach the stiles to the rails.
Saw off the wood to make the panel. On each side, the panel should be 3 ? inches shorter than the door opening.
Attach the panel to one rail and stile. Glue it securely with wood glue by pressing the corners of the frame. Slide the panel into the other rail, and glue it to the first stile. Slide in the remaining stile, and glue the unsecured joints.
Use polyurethane gloss to apply finishing touches.