Yeah the instructions didn’t make a ton of sense for me either. Basically I R.A. a tape line down the top, cut a straight clean line with an angle grinder and did the same on the bottom. I then put the tip on the wing and eye balled if the tip was square to the wing. If it wasnt square I trimmed more of the side that needed to be brought down to make it square. Don’t make your first cut at the scribe line so you have enough material to work with.