I'd put the horn on the control surface in the desired place, make a hole with a very small flat screwdriver (quite easy to go through balsa), put one screw in and screw it into the other piece almost all the way in. Then realign the horn, drill the next hole with the same screwdriver, screw the second screw in. The control horns I'm using have 2 holes only, so the next step is to tighten the screws but you could make as many holes with the small screwdriver as holes are on the control horns.