You just have to imagine that you're flying a real plane and need to use the ailerons of level the wing.
So if it's trying to turn right, you need some left aileron to correct. On the foamie, it may be assembled a little out of whack, so steaming and heating will only be a temporary fix. I would suggest making a balsa trim tab and adding it to the outside wing tip. Without seeing the plane in flight, I'd guess 1/2" x 3" would be a good size for a start.
Yeah, right, inch, not foot, unless you have a 120" wing.
The advantage of a trim tab is you can tweak it without a heat gun.