The flap is put together by drawing a centerline down the three flanges of each rib (top, bottom and front). I did this and then drilled the ribs to the spar by lining up the centerlines. The bottom skin was then clecoed to the spar and the holes in the ribs were drilled the same way.
I then took out the clecoes and clamped the hinge to the top skin. I lined up the hinge with the outside edge of the skin and positioned it so that there was 3/8" between the hinge axis and center of the hole in the skin. I then match drilled the hinge using the holes in the skin to guide.
I think the instructions could use some improvement in the section on the flaps. I didn't find anything on how to laterally position the hinge or how much to cut off.
I peeled more plastic, deburred the holes in the hinge, skin and spar and then dimpled the holes in the skin. I then started countersinking the holes in the spar.