Start by placing this magnetic level on your scope base. It's best to
also contact one of the scope rings. Position your rifle on sandbags or use a bipod, and focus your scope on a known horizontal (or) vertical
straight edge to use as a reference for your crosshairs. When the scope level shows your "rifle" is perfectly level, you can rotate the scope until
the crosshairs are parallel to your straight edge. You can also hang a string with a weight on the end to use as a perfect vertical reference.
Then tighten your scope rings, sight-in your rifle and you're ready to go.
Another reason for getting your crosshairs perfectly level is just to "know" that they're exactly right. When I pick up a rifle and see the crosshairs are crooked . . . it sure doesn't help. Absolute confidence in your rifle is one of the most important factors in good shooting. This is one more way to help your concentration.