Well, this is pretty simple and basic, but it has worked for me so far. I put the rifle in a cradle and put a level across a scope ring base. I shim the cradle until the scope base is level. I place the scope in the rings and place the level across the top turret and level the scope. I watch the level while I tighten the rings as the scope will move during this process. I tighten first one side a bit, and then the other until all screws are tight and the scope is still level. Another method I have used is to again level the rifle in a cradle, and then hang a plumb line out 100 yards or so. Align the verticle cross hair and tighten rings as described. Much more monkeying around this way. Simple, but it has worked for me.