Attach the Slats
Position the narrow slat centered on the assembly. It will extend beyond the end rails by roughly 4 inches on either end. Use a square as a guide during this process. Mark the position of the narrow slat on the end rails with a pencil. Remove the narrow slat, apply glue to the marked area, replace the narrow slat and attach it to the end rails using 2½-inch screws. Mark a line on the end rails ¼ inch from the narrow slat on both sides of the slat.
To make painting easier, paint the assembly now except for the tops of the side rails, the tops of the legs and the areas on the end rails on the other side of the ¼-inch mark. Paint the edges, ends and one face of the wide slats.
Apply glue to the unpainted areas on top of the side rails, legs and end rails. Place the wide slats on the assembly, making sure the spacing is ¼ inch and that the ends are flush with the narrow slat, and attach them using 2½-inch screws.