what's the best way to double bevel flying surfaces? I'm going to be building a Giles202 and want to get the surfaces so I can do 3-D

Use a razor plane and put a 35-40 deg angle on both the control surface, and it's associated "wing". This will give you a 70-80 deg total deflection when you hinge them, (If you get a perfect gap) and maybe a little more in reality. This is way more than enough for 3D, and will actually probably be more than your servos are capable of moving, unless you have really long arms/horns and high torque servos.

you can also glue a small triangle stock to both to get the double bevel easly

Whether you plane it or sand it, the trick is to mark a centerline on both surfaces first so that you have a target to shoot for.

I glue tri-stock too. For the gP G-202 there is a trailing edge cap for the wing. Replace the square stick with tristock that will be thick enough for the ailerons to line up with the center section of the wing. Replace the leading edge of the aileron in the same fashion. For the tail, just add tri stock, and extend the front of the airbalances to make the leading edge of the surface look right. Good Luck-

All of the above methods will work fine. Be aware that surfaces become more prone to flutter with double bevels so be sure to seal the hinge lines.