Attach one of your long boards to one side and one of your 3' - 6" boards to the bottom of your small square frame. Screw this in with pairs of screws at the bottom, the top, and in the middle.

Note the way the pieces come together at the corner. The 3' - 6" board projects past the small frame that you built in the previous step by 1 1/2" on each side. The long board that goes up the side then meets flush with the butt of the the board across the bottom.