With the parts primed I first clecoed the doublers and bottom rib to the spar and riveted them in place. I’d never riveted before so this was a nervy experience but turned out to be real easy and straightforward.
After this I attached the remaining ribs and worked my way up the spar until all of the parts were secured to the spar.
One open question I have is whether the horn should be attached at this point. It doesn’t mention it in the photo manual but it does show it on the plans.