I mark the center line (thickness-wise) of the aileron, rudder or elevator and then take the measurement of half the thickness and mark it on each side of the control surface, then sand all of the material out between these lines and you get a nice bevel. On one plane i built, I cut a piece of 2x4 at a 45 deg. angle lengthwise and used that as a guide for my sanding bar, it was easy and worked great! The last plane i built, I marked out the measurements and used the sanding bar as Ken noted earlier, turned out good too, but required more work than using the 2x4 guide.