Attach back supports
Cut the 18-inch-long back supports so they taper from 5½ inches at the bottom to 2 inches at the top.
Align the back edge of the supports with the back of the seat and attach the supports to the post by drilling pilot holes and driving 3-inch screws. Attach the center back support by driving screws up through the seat from underneath.
More: where to start and what you'll need