If it's a straight wing, ie all the ribs are the same, I use tracing paper to make two templates out of 1/16 ply then sandwich balsa rectangles big enough for the ribs and the correct quantity. You can pin them all together or use long bolts. Then you just plane and sand to the templates and voila all the ribs are cut out. you can also cut all the notches for the spars at the same time. I hope this helps.
Bill