Brief Description: LEFT Wing - lower aft skin (more photos)
To make sure the holes in the skins align with the ribs, it helps to have a centerline drawn on all the ribs. There are "tricks" for doing this, but I didn't find them necessary. We're not building a Swiss watch!! I just drew the centerline on the ribs by holding the pen at the center of the rib, then using my middle finger against the rib to hold that same pen position as I moved down the rib. You can just see my finger in the shadow of the pen.
Got all the pilot holes in the rear spar to set the skin, then started on the center rib and worked to the main spar. Moved to alternating sides of that rib, working rear to front.
No tricks to draw the centerline on the ribs; just my middle finger against the rib while drawing