Not so much a science, as an art. With the right scope, bases and rings, it can be very simple. With the wrong bases and rings, it can be a nigthmare.
That said, its easily done if you take your time and pay attention to detail. Some details on what you have, what you're trying to mount it on, and what you're mounting it with would help.
Ie: Bushnell Elite 3-9x40, Savage model 12vs, Burris medium hight Sig Rings? Piece of cake.
There's enough guys here that can talk you through it.
The key points are:
Bases square, tight and degreased
Eye-relief set properly for the scope
Reticle level and plumb
Scope mounted to not hit action parts.