Place the rifle in a vise if you can, I use a Lohman SightVise, make sure the rifle looks perfectly aligned vertically, not canted to one side. Hang a heavy string 10-25yds away with a weight on the end so it works as a plumbob, align the vertical X-hair with the string and tighten the ring caps being careful to not allow the scope to rotate in the rings which some rings are worse for than others, Weaver rings in particular.
Tim