Start the seat assembly by joining the outer frame with glue and 2-½” screws using clamps along the way to help square things up. Add a taper to the back of the uprights by striking a line 1″ over from the top notch and 6″ down the upright.
Seat supports screw
Put the back in place and join it to the seat supports with a 3-¼” screw through each upright into the seat supports.
Repeat this process for the other side and before moving on drill holes at the bottom of each assembly for attaching the base of the armrest later. For the inner supports, space them evenly with the center of the boards 23″ from the outer edge.
Clamping some temporary blocks to the sides to position the seat which should be 12-1/2″ to the top of the seat. On the back side use two more screws to secure the outer pieces together and put another fastener into the back of the inner supports.