Mount the scope base and lower half of the rings (if the Redfield type, then install the top half of the rings on an alignment tool or bar to align the rings, then remove them.) Place the scope in the rings and adjust eye relief to the maximum comfortable distance then mark it with tape for reference. position the top of the rings and srews, but do not tighten them.
You can place your rifle in a rifle vise or cradle. most folks don't have these, so just use sand bags or bags of shot or a good substitute that will hold the rifle in shooting position without touching it.
Then place a square target in front of the rifle on a target backer that has good color contrast about 25 yards away. use a level to get the target edges absolutely square.
while the rifle sits in the rest, move it to get it square using a level or plumb and a scale or ruler.
Now look through the scope and rotate it so the elevation crosshairs follow the same plane as the top of the target. snug the screws and visually recheck the alignment and the tape marking your eye relief.
Repeat this process until your satisfied with the reticle alignment.
(If rings are Weaver type, tightening the screws in the rings will rotate the scope body as torque is applied. you have to allow for this movement before you begin the tighten the screws. only experience can tell you how much to allow.)
mcrik