You can use your miter box to cut angles on the ends of the curved pieces, but you’ll have to steady them by supporting them with one of the scrap concave corners cut from the 1×12. Place the straight edge of the concave scrap against the fence and nestle the curves. Then sight along the blade and adjust the angle to cut along the line.Use this same technique for cutting the angles on the ends of the curved door frame tops (D) as shown here.
Assemble the door frame with pocket screws as shown. Then cut the curved top (D) in half and cut angles on the ends to fit. Attach them with pocket screws as well. Place the assembled door frames in the face frame to check the fit. Plane and sand as needed to allow a 1/8-in. space around and between the door frames.
After assembling the face frame, flip it over and screw on the cleats (B2 and C2; Step 5). The cleats overlap the joints to add strength and serve as a nailing surface for the floorboards and side panels.