Step 8: Cross Members
Ideally, they would match the dimensions in the plan, but even a 1/4" variance in your build would cause them not to fit properly, so we cut them custom to the final assembly of the chair. Regardless of variance in your chair, the two cross members should be the same dimensions, so make one and use it as the template to cut the other.
The cross members are angled at about 32 degrees, so make the first cut (noted in the first diagram) at that angle (the interior of the angle is noted on the diagram as 122 degrees, which is 90 + 32). Then measure up the cut edge 2" and draw a right angle line - make this cut second. This end of the cross member will fit under the seat (see second diagram).
It would be really nice if you could dry fit this piece somehow and mark where the next two cuts at the other end of the cross member go, but that isn't possible. After trying all sorts of methods, I finally settled on just making the third cut (which butts against the footrest side) a little long, dry fitting, and trimming down until I got the proper fit. The goal is to have the bottom of the cross member even with the bottom of the seat and the top of the cross member even with the top of the footrest.
Once you have them cut to size, glue and screw them into place. The back cross member goes all the way back, and can be screwed from the inside to the back of the seat, and from the outside to the side of the footrest. The front cross member's position isn't critical, but should be as far forward as you can put it, but behind the front legs and comfortably out of the way of your feet.
Let the glue dry, grab a cold drink, and have a seat.