Washout Tutorial Needed
An easy way to do it is to make a spacer that runs the entire length of the wing. For a flat-bottom wing, it would start at Zero and end at whatever dimention of washout you want (Let's say 1/4").
So if your wing panel is 26" long, cut a scrap piece of 1/4" balsa 26" long. then make it taper down to nothing. Get your ribs in place on the spars, then place your tapered piece so that the thin end is at the root and the thick end is at the tip. Place it just under the rear ends of the ribs so that it is lifting the tip ribs more than the root ribs. Now glue the ribs to the spars, add shear webs, and LE an TE sheeting. The washout should be now a permanent part of the wing.