I hang a plumb bob from the side of my shed and then set my rifle up on a card table on sand bags (any stable rest will do) at about 20', which is the distance I can clearly see the plum bob string through the scope on lowest setting. I remove the bolt and loosen the rings enough to allow free movement of the scope. I then align the rifles bore to center on the string by eyeballing. I then align the scopes windage reticle to same plumb as the plumb bob. Then check the bore alignment and scope alignment again until I'm sure all is good. Then I tighten down the ring screws alternating back and forth and rechecking alignment. I have an anti-cant, top ring, bubble level and I make sure it reads level also by alternating the ring screw tightening.
Then I take a scrap peice of plywood or cardboard about 4' high and draw a line on it and also a perpendicular cross line halfway up as my POA. I mount the plywood so the line is plumb and sight the rifle to center on the verticle line and confirm with a 3 shot group @ 100 yds. I click up 20 MOA (about 21 inches) and fire another 3 shot group looking for it to center on the line. I click down 40 MAO (20 MOA below zero) and fire another 3 shot group. I then click up 20 MOA (back to zero) and fire another group. If all groups are centered on the line, I'm good to go. If the scope is canted your top group should be offset from the line about the same amount and in opposite direction as your bpttom group.
The last time (and first time) I tried this, my "eyeball" set of the scope was perfect according to the fire test results. I also found that the scope tracked perfectly... perfect distance based on MOA adjustments and perfect back to center. Killed two birds with one stone.