Keeping the frame in its current orientation, next we need to attach the seat support. This was the 17" 1x2 with the two angled blocks glued on. There should already be a mark at 9 1/2" on the side rails from a previous step. Using that as a reference, measure back to the curved ends 3 1/8" and mark it. Why 3 1/8"? The seat frame is going to slot between the back bottom rail and the seat rest. The back bottom rail is 1 1/2" and the seat frame is another 1 1/2", giving a total of 3" plus an 1/8" for wiggle room. Could we have just measured from the curves? Maybe, but your carving probably changed the length of the stick, so it is better to go from the fixed reference point of our 9 1/2" mark.
Attach the seat support with the angle blocks on the new marks. I used my glue and a brad nailer, but screws could work too. Just make sure to drive your fasteners-of-choice into both the main spar and the angled blocks.