Lay a long side on the flat, and use a 1x3 turned on edge to mark off space at the ends, where the short sides will be attached. Apply wood glue along one edge of the long side between the marks. Press a ledge into the glue, flush with the edge of the side piece. Use the nail gun to drive 1 3⁄8-inch nails a few inches apart through the ledge and into the long side. Repeat for the three remaining long sides.