Draw a line down the front of the aileron leading edge that is about 1/4" down from the top. Draw a line down the top surface of the aileron that is 1/4" back from the leading edge. Draw a 3rd line on the bottom of the aileron leading edge that is about 1/2" back from the leading edge.

Using a razor plane (Master Airscrew is probably the best you can use) start to "connect the lines" by shaving off the balsa. You'll have a 'V' shaped leading edge when you're done.

The hinge line is above the center point of the leading edge for several reasons. It looks better, it allows the use of any type hinge and it provides enough clearance for "more up than down" aileron throw. This will make your flat bottom wing roll a lot better.