Start by marking where you want the cat flap to be. Then, drill through the corners, keeping the drill horizontal. Drill the screw holes marked on the cat flap. Next, insert a jigsaw with a steel-cutting blade into one of the corner holes. Cut on the outline, then insert the exterior half of the pet door. Match the thickness of both doors, then fasten the flap. Lastly, apply caulk around the edge of the cat door.