What kind of bases and rings are you using?

If leupold style the the best way is to get a 1" dowel rod and cleanly cut two pieces about six inches long each. Place each one of these in each ring and tighten down just like a scope.

The purpose of this is to make sure that your scope rings are alligned properly and to provide you with leaverage to twist the front dovetail ring to properly line up with the rear. Never use your scope for this. The rear base will have a windage adjustment for the ring so that you may move it from side to side with a large flat head screw driver. Adjust the rings so that the dowel rods line up perfectly with each other.

When the two pieces of dowel rod line up perfectly in the center of the rings you are done.