I build the back of the frame first. I first made the small rectangles by screwing the pieces together. Then the large outside rectangle, again with screws. The small notch on the left side is for the plugs to come out. I also put a notch on the MDO frame so the whole frame would still fit flush against the window. Just line up the back boards (same luaun as the front) and shoot a bunch of nails. When you are done it's relatively sturdy.