Cutting a notch in the spar to accept the shear webbing results in a tremendously strong "I-beam" stucture for the wing. The best way to do it would be to use a router table with a 1/16, 3/32, or 1/8 bit and route the notch into the spars. Unfortunately, most of us don't have such a setup. I tried it with a Dremel tool and Dremel router table, and the results were marginal. I've also used a radial arm/table saw to rip the notch; again, this works if you have a saw and a narrow kerf blade.

It's also, for the most part, unnecessary. If you were flying a 3D ship, you might need the added strength. You'll be fine if all you do is glue the ribs to the bottom spar, cut the shear webs to the distance between the spars, glue in the webbing, then glue the top spar to the top of the webbing. Most wings are built in this fashion. You can even build the wing, then glue the webbing to the back/front of the spars. On a Kadet Senior, any of these methods will give you the necessary shear resistance.