Make sure get the correct one first. The front left rivet rear trunion rivet locates the rail. There is a difference between the AK47 and AK74 style rails.

1) Remove the left front rear trunion rivet
2) Place the rear hole of the scope rail in line with the rivet hole above
3) Useing calipers, measure down from the top of the receiver to the flattest part of the scope rail. As close to the rivet as you can get
4) Lock the calipers
5) Go to the rivet hole on the other end. Use the calpiers to get the same distance to the top of the rail
6) Clamp in place
7) Drill holes and rivet in place

A lot easier than it sounds. Hope it helps,

Mike