When I mount a scope, I like to boresight it before I take it to the range. I use a boresight to get on paper, not as a definitive alignment procedure. I simply want to make sure that the bore of the rifle and the scope are pointed in the same direction.
Some folks make devices to assist in boresighting a rifle, but I trust the method the Army taught me when we were boresighting tanks prior to taking them to gunnery.
I apologize for my photography skills, but these pictures illustrate the method I use to boresight a rifle. The best part of my method, is it doesn't cost anything.
This fuzzy image is taken down the bore of a .30 caliber rifle. (I was having depth-of field problems.) What you can see is a blue circle in the center of the bore. That's a hose hanging on the wall of my neighbors garage, approximately 75 yards away.
This image is through the scope, showing the reticle closely aligned on that same hose hanging on my neighbor's wall. I can be reasonably assured that the scope and the bore of the rifle are looking at the same spot. I'll verify boresight at 25 yards, them move the target out to 100 yards and begin serious sighting.
It's quick, it's easy and it doesn't cost a thing. I optically centered the reticle before I started mounting the scope, using the method Junior talks about over at Castbullet. I got lucky with this installation as the scope is centered over the bore and the reticle is optically centered. I didn't have to shim anything, nor make any adjustments to the scope after mounting. This is probably going to be a very accurate installation.