Quite exciting this bit, since it actually looks like a real plane! Vans say cleco, clamp and line up using a #30 drill bit. This all seemed to work, but I also propped up the front on blocks of wood.
A big BOLD note in the manual says:
NOTE: It is critical to drill the holes perpendicular to the Vertical Stabiliser Spar.
So I taped on a protractor with an edge aligned with the spar face. I also stuck a set square on with some double sided sticky tape. I was able to drill 90º horizontally by aligning the drill bit with the set square, and Coral helped by checking the vertical 90º as I drilled. I decided to drill in two stages, starting off with a smaller drill. There are several layers of metal to drill through, and I applied some BoeLube to help things along through the steel layers.
As holes were drilled AN4 & AN3 bolts are inserted to ensure alignment. I nipped them up with some wing nuts I had bought for the purpose. Amazon did a nice little set which included 3/16-32 and 1/4-28 wing nuts.