1->By pressing the drift button (L/R), your character will keep going in the same direction while turning. You don't need to turn at all. 2->This turning goes until it reaches the maximum angle, and caps. The time needed for this to happen is different for each character. 3->If you release the drift button just when the character reaches that cap, you get a free boost.

Bear in mind that this mechanic is extremely situational, and should be used carefully. Here's an example of three boosts performed consecutivelly: