RE: How the f*ck!
Practicing.
Most of the 360 no-scopes accross map you see are either set up or extreme luck.
In private matches, or playing Drop Zone (MW3), I'm somewhat good with quickscoping, but I only do 360's or 720's and things like that on private matches with my friends.
Also, in the montages, you see the quickscopes and noscopes and stuff, but you don't see how many times they tried to actually do that.
I'd send you the video of a commentator that found himself playing against a clan of quickscopers while he was the last man in Search and Destroy, everyone of the clan jumped and did like 360 noscopes trying to kill him, and failed, until someone got mad and simply quickscoped him; but I can't seem to find the video.