Build the small box as shown in Image 1 at left.

You will be screwing through the 3' - 3" boards and into the 3' - 6" boards. Pre-drill two holes through each end of the 3' - 3" boards so that you don't split them. Use 3" long screws for the attachments.

Once all four pieces are attached together, square the frame by measuring the two diagonals and making adjustments until they are equal. Then, brace the frame by screwing a diagonal piece onto the back as shown. Double-check your diagonals and make sure that the frame did not come out of square as you braced it.