Glossary
Crosshair placement
In one line: Always keeping your crosshair where an enemy's head will appear, before you see them.
What it means
Crosshair placement is the single most important aiming habit. Instead of hunting the enemy once they appear (moving your mouse far and fast), you keep your crosshair already at head height and on the edge of the corner they can come from. When they show up, you only have to click or make a tiny correction.
It applies while you walk too: your crosshair does not look at the floor or the sky, it follows head height along walls and corners. Players who do this well look like they have incredible reflexes; in reality they move their mouse less than everyone else.
An example
You are entering a corridor that turns right. Instead of looking at the middle of the corridor, you keep your crosshair on the right edge, at head height. When an enemy steps out of the corner, they are already on your crosshair: one click.