Most scopes are flat on the bottom, so one easy way to get it level when mounting, is to use a feeler gauge. This only works if your rings are attached to a picatinny rail though, unless there is a flat surface below the scope, such as some revolvers have.

Just tighten your rings enough to where the scope can still rotate easily, slip the thickest and most rigid feeler gauge underneath the scope and put one edge of the feeler gauge in the bottom of the picatinny rail and lift the other end up against the bottom of the scope and it will level it perfectly. Then tighten the rings.