I just did this you have to mark 3 inches back towards the wing tip with a felt pen on the bottom wing skin to the open rivet holes that should be open. It is the last row of rivets and there is several of them and you will have to mark them with the wings off. Then when you mate the wings you can make the mark back 3 inches and drill the holes in the bottom fuselage skin then take the wings back off and add plate nuts. I drilled them with a very small bit then went up to the right size. If your rivet holes are not open you will have to drill them out. You could use a hole duplicater if you already have the wings mounted. The mark is just to help you locate where the holes are in the wing skin.