In order to build them, you must use them, in order to use them you must activate them. Here are some flute activation exercises that you can do to grow a "backpack booty".
This is my favorite one!
Place a ball between your hip & the wall.
Without your hands, roll the ball a couple inches up the wall
and return to neutral to get those puppies firing.
The of course once they are awake....squat, lunge, deadlift & drop it like it's hot.
SQUATS: the difference between having a butt & having an ASS.