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Mount the gun in the shoulder, as you would normally fire the rifle. Keep in mind that to adjust the eye relief, the distance changes depending on which shooting position you use, standing, prone, off a bench, etc. If you see a dark ring around the edge, this means the scope is too close or too far away from the eye. Slide the scope forward or backward until the view through the scope is full and clear. The scope also has an adjustment, but do this first.
If you are mounting a scope on a heavy recoiling rifle, make sure there is enough clearance between you and the scope. Now, rotate the scope in the rings until the crosshairs are nice and level. Yes, they sell a tool to do this too, but you can do it by eye too. Hold the rifle level, and align the scope to match. Make sure the scope's windage knob is on the side and the elevation is on the top.