1.) Remove the scope from the rings.
2.) Leave the rings top caps off.
3.) Clamp the rifle in a vise.
4.) Use a good 4' level laid across the ring bottoms to get that rifle level side to side. Fore and aft do not really matter. Here, it is important to have the butt of the rifle facing a wall about 4 feet away.
5.) Place the scope in the rings and attach the caps but loose enough to be able to rotate the scope.
6.) Tape a flashlight to the barrel shining towards the butt of the rifle through the scope.
7.) Adjust the magnification and parallax until the reticle is nice and clear on the wall.
8.) Rotate the scope until the vertical is plumb. You can use the level or hang a bob.
9.) Torque the cap screws while making sure the scope does not rotate.
EDIT: to say that if you have a tripod, you can use it instead of a vise and it's also easier to get leveled.