I have a small (about 2" long"), cheap plastic bubble level...a bubble is a bubble, nothing fancy needed here- with a FLAT base (must have).
I loosely set the rifle in a Winchester (wally world) sight-in rest, place the level either on the picatinny base, or across a flat spot on the top of the receiver. Must be 90 degrees to the bore, rock the rifle until the top of the receiver is level, and clamp it tightly in the vise.
Lay the scope lightly in the rings, place the level (again, 90 degrees to the bore) across the top of the elevation knob, with or without the cap- whichever gives the most solid surface on which to rest the level. Level the scope, and begin to tighten one of the rings.
Place the level back on the mount/receiver, make sure the receiver is still level, back on the scope to make sure it hasn't moved either. Always takes some back and forth tweaking, as one or the other is going to move slightly.
Once I've got a couple of screws in one of the rings and all is well, I'll place the others (with blue loctite), then go back and loosen the first two "temporary" screws and loctite them back in place.
You can also level the rifle in the rest outside, and sight it against something you know is plumb (or should be
) like a corner of a neighboring home- and align the vertical crosshair with the corner of the home. No matter which way you go, it is critical to have the rifle's receiver level as well as the scope or you haven't accomplished anything.