Remove old cabinet doors from the cabinet with a screwdriver. Place the door in a well-ventilated area. Sand the door to remove the existing finish, using an orbital sander and 60- to 120-grit sandpaper.
Measure the height of the door. Cut two strips of 1/4-by-3-inch board to this length. Apply a bead of glue to the back of the board. Align the board with the top and side of the cabinet door and clamp the board in place with wood clamps.
Measure the distance between the wood strips. Cut two strips of 1/4-by-3-inch board to this length. Apply a bead of glue to the back of the board. Align the board with the top and side of the cabinet door. Clamp the board in place with wood clamps. Wipe off any excess glue and allow it to dry for 24 hours.
Sand the door with an orbital sander to unify any discrepancies where the wood joins. Use wood filler if necessary and finish the door to your taste.