Easiest way to do it if you have a front sight post is to center the reticle horizontally on the front sight post. You then center your reticle vertically once you dial your hop up in like Thunder Cactus has said. I use a pop can at about 80ft that way I have a very audible *ting* to know I am on target.
If you have no front sight post, use a tooth pick. By this I mean find the center of your barrel's bore and place the toothpick vertically at this center and make sure its high enough that it is in your sight picture (doesn't have to be up to where you do is, just high enough to see in the lens). You then adjust the same way horizontally as you would with a front sight post sitting low in the sight picture. Once its centered you proceed to adjust the hop up accordingly. For this process you don't even need to fire the gun, so taping the tooth pick in place over the muzzle is not an issue (an old MacGuyver trick).
If you are still shooting left/right of your target, have a friend stand downrange to spot you and tell you if you are shooting left/right/high/low and make adjustments accordingly. This is what I do every time just for extra assurance. Its not dangerous to do this as these are BBs, not high velocity rounds, so as long as he/she has eye pro on they can stand literally a few feet from your target to get a good sighting on where your rounds are landing.
Sounds complicated, its really not.
Guardians of Asgaard
NO EGO, ONLY HARD WORK
Primary - Z-Custom VFC ZTW
Secondary - KWA ATP
Last edited by Zack The Ripper; July 28th, 2013 at 12:53..