They left front rear trunion rivet sets the heigth of the rail. Here are the speps involved:
1) Remove left front rear trunion rivet.
2) Place optic rail on receiver so the rear hole lines up with the removed rivet hole.
3) Place a rivet throgh the optic rail, the receiver and the rear trunion.
4) Measure from the top rail of the receiver to the top rail of the optic rail, as close as possible to the rivet. Calipers are best for this, but you can get by with a steel rule.
5) Measure the front of the optic rail the same way and adjust so it is as close as possible to the dimesion of the rear.
6) Once you have it where it needs to be, you have two choices. Mark the other holes or rivet the rear one down, remeasure the front, adjust if needed, drill the remaining two hole and rivet them.
If you are useing screws, just tap the trunion hole and follow the other directions useing screws instead of rivets.
Make sure you get an optics rail for the rifle you are useing. The front rear trunion holes on the 74's and 47's are different and you will not line up with the trigger/hammer holes. Take your time and you should be alright. The rail doens't have to be perfect, but it needs to be as close as you can get it. When I was building them for a living, I had made up a gauge to place them where they needed to be. Hope this helps,
Mike