"How do I get to Carnegie Hall?" "Practice, practice, practice."
The same holds true for shooting accurately quickly.
Learn to shoot super accurately and the speed will come with practice. Just hammering the trigger as fast as possible does NOT guarantee worthwhile hits. This ain't Da Hood where yooz Spray & Pray dat you getz Da Homeyz.
It is not only unwise, but dangerous to shoot faster than you can hit the target.
A "hammer" is one flash sight picture, two quick presses of the trigger. With practice this is quite effective at closer ranges with shot splits down in the .15 second range easily obtained. Muscle memory plays a major part in the effectiveness of this technique, and you get that through lots and lots of practice.
A "double-tap" is two seperate and complete sight pictures with two presses of the trigger. Shot splits for this technique can range from .25 seconds apart to as long as 1 full second apart, which is an eternity.
I can't stress it enough, practice, practice, practice. Buy a reloader and reload a ton of cheap practice ammo. Get a timer, join a range, get your trigger time.