This can be frustrating. I use the horn as a template/guide to drill the holes. Start by drilling one hole, put a screw in that hole, drill the next, put a screw in and so on and so forth. I attempt to hold the drill perpendicular to the surface. If needed to get alignment, I chase the holes with the next larger drill bit. I agree with Minnflyer the holes themselves are less important then the clamping action of the horn and backplate. Generally, I find two holes will align enough to get screws started. If the third or fourth hole is misaligned, I see how it is misaligned, remove everything and re-drill the hole making a correction for the misalignment.

Once I know I can get the screws started, I remove everything again, cut the covering from under the horn and backing plate, take a T-pin and make many perforations into the wood from both sides of the control surface in the location of the horn and soak this area with thin CA to harden the wood. After the CA kicks-off, I will chase the holes out again and then finally permanently mount the horn.