When you place the skin back on the wing skeleton you should see the rib centerline through the rivet holes (if you were careful when marking the skin and using the pattern!) It really is not hard.
Again, it works amazingly well. I marked and drilled a bottom skin and the rib centerlines lined up all the way across the skin. Never more than a half off of the centerlines.
Well, that’s my story. If you have any questions feel free to email me. I’ll try to ‘splain it better than I did here.