I start by deciding where that darn hole needs to go and give er a good wallerin!
In all seriousness, I do start this obviously in a spot that I need a hole made usually on an aircraft body or even a spot that I cant even see! If I know the hole location and size I choose a smaller diameter rare earth magnet (round type not the square)
Place the magnet on the spot desired. Place another magnet on the opposite side of the surface so that the magnets attract and "hold" one another to the exact spot to be drilled, or wallered if you prefer.
Use a fine tip sharpie or marker to mark your spot and remove the magnets.
Drill your hole according to the size desired.
This works excellent with blind nuts as well. If you cant see them darn nuts how do you make a hole that won't look like a kindergartener made it? This is your method you don't even need the second magnet for blind nuts.
Anyways hope this tip helps someone!
Sincerely,
S.N.