First thing you need to do is work on a level surface.

Now get out your ruler and your pencil and do some layout. Mark where you want the control horn to go. Put marks where all the screw holes go. Mark the clevis--mark whatever you need to do so you KNOW where every screw and clevis is going to attach. This is not the time for "thats about right"

Now you set your control surface on the bench and block up the trailing edge (brown scrap wood) until the centerline of the control surface is level. Notice the red line in my drawing. That red line should be equal distance from the bench on both ends of the red line. Then you know it's level.

Now you set your drill press to level and do a test run through scrap 1x2 and make sure the head is indeed drilling a perfectly vertical hole in the wood. Not slanted off on the X axis by 2 degrees or tilted forward on the Y axis 3/4 degree. Perfectly straight up and down.

Locate the holes for the control horn screws. You should already have the holes laid out.

Drill the holes.

It's all in the setup. Takes time, but done right, it will make you a better modeler.