After you've gotten flicking down, practice no scoping. I got this technique from playing CoD4 :]
First, you have to know where your crosshair is, It helps if you know how to flick quickly, use a crosshair hack, or can mentally picture a crosshair in the center of your screen.
Second, you have to shoot while pressing SD(Backwards + Right) at the same time and also while flicking/swishing your mouse.
Lastly, practice.