Original Author: AL
When: The time an opponent is fixed on finding/stalking his/her target. Or when he/she is under fire and is fixed on shooting back. (When the opponent is alone)
What: Crouch, aim, shoot.
Why: Your enemy is vulnerable, and is distracted. It is the perfect time to take the kill. This technique will help when up against average pro's.
How: Only find the opponent that is sure they want to kill their target. Creep in a corner, box, crate, or anything you can use as cover. Wait until they are firing, or when they are reloading, because that is when they are most vulnerable. Aim for the torso and above. That will ensure that you get at least one headshot, and will ensure a fast-kill.
Bonus: When you master this technique, you won't have to stalk your prey and wait until they are at their most vulnerable point. You can just pop up and kill the opponent on sight. Also try using the Bunny Hop.
How to practice: Noobs, obviously, are good for practice because they are slow, uncertain, and take forever to shoot back. Go on a large server with plenty of other targets to distract the enemy. Watch the other players while waiting to respawn, and find areas containing easy targets. After spawning, hunt them down, wait for your opportunity, and then blindside them. Repeat ad nauseum. Really, you should not have to practice this at all; it is pretty simple.