Use a miter saw to cut parallel 30-degree cuts to the ends of the backrest supports, per the cut list. Slide the side support between the leg and the seat base so that the bottom is flush with the underside of the base's outer stringer. Clamp it in place and drive 2⅝-inch screws through the support and into the crosspiece. Repeat on the opposite side.
Tip: If you prefer a straight backrest, square-cut the supports and reinforcement block instead of mitering them.