Leveled saw horses and sqared ribs to main spar. I used strips of aluminum that I attached to the aft spar and main spar to hold the structure square. I probably didn't need to do this as the process of aligning the aft skin to the rear spar and center of the ribs really self aligns the wing box, but this probably made it easier.
I also ensured there was no twist in the wing by using the plumb bob check from aft to forward machine holes in the aft spars.
