If you have done a half way accurate job of tracing the rib outlines with a fine line marker, from a master pattern rib, then it should be an easy job to trace your lay out lines with a #11 blade. You should be using fairly light grade balsa, right? Now stack all the rough cut ribs together and see if you can press a hunk of spar material into your lined up notches. Work the notch with a file if it is too tight. Now do the same thing with the other spar notches while the ribs are still indexed on the first spar that you fitted. Now with all the ribs solidly pinned together by the scrap spars holding them together, you are ready to sand all the ribs as one unit, to a point where they are even, especially even in length. If you slightly notch the L.E. and T.E., [where the ribs intersect] it makes for a better glue joint.