Choose a player with the red wing looking symbol (High Flyer) with three little bars underneath the symbol. The bars represent his level, and all three have to be lit up the color of the ability. Have him take the ball inside the painted box underneath your basket. Hold down the LB button and while holding it down move your right stick in a complete circle. Your player will do a crazy dunk, and if you make the dunk the achievement will unlock.
If your player has more than one ability (such as Kobe Bryant) hold the LB button and tap RB to change abilities. If you press both at the same time you may accidentally throw an alley-oop.