This is quite difficult to do without a lot of practice and craftsmanship. I suppose if you had a disc sander and a jig, you could make it work, but...

Why don't you simply buy some of the pre-made aileron stock from your lhs? It comes in varying sizes that you can trim to suit.

If you are forced to do it by hand, you'll want to draw a centerline down the trailing edge of the piece you're sanding, so you'll know when it is "even."