Starting at the top, place the first 1x4 board against the backrest supports, flush with their tops. Screw the board to the supports as described in Step 12. Secure alternating 1x2s and 1x4s beneath the first one, using ¼-inch spacers between them during installation. Cut the 1x4 cap to size so that it fits between the back legs. Apply a thin bead of construction adhesive to the top edge of the first 1x4 backrest board and the tops of the backrest supports. Set the cap in place and secure it to the supports with 1¼-inch screws.