As mike mentioned, you need to describe the construction of the wing.

Guessing that it is basic rib construction, I would suggest you start by deciding how long you want the aileron to be. Then decide where you want it located (at the end, in the middle or wherever) Then when you have this all figured out, if the wing already has a solid trailing edge from root to tip, you will need to cut the decided area out. When you cut this area out, you need to make sure that there is enough wood to put hinges into. If there is not, I would cut out another 3/8" into the wing and then glue in a piece of 3/8 balsa stock the entire spane of the aileron. Cap off the ends of the section you have cut out and you should be ready for the aileron installation.

Here is a picture of a HERR STAR CRUISER wing that I put ailerons on. This kit is designed without, but I did exactly what I described above. You can see that it is a fairly small aileron and for this reason, I moved it toward the outside edge, but not all the way. The only reason I put the ailerons on this plane was to have better control for landings.