I just cut the flaps out of a 1/72 f4f wildcat. used the #11 exacto blade as quoted in the above posts, worked fine if a little time consuming, but a good acurate cut is priceless. If you got the room you might fit a razor blade in the cut and go back and fourth. In larger scales a scriber can work, but I start with the exacto anyway.
Another idea (if the top & bottom wings a seperate) is to drill a hole with a pin vise and bit, insert a jewlers saw blade through the hole connect it to the saw handle and gently saw your way around. All depends on wht scale your in I guess. Also any method you'll see here will allow you the possibility of screwing up the wing or whatever, so have fun, don't make two of yourself (was mom's favorite quote) and don't cuss all of us if it goes south!!!! Ha Ha Ha Ha Haaaaaaaaaaaaaaaa!