Well this is going to big post covering the basic for all the guys trying to get a bigger butt.
First things first, take note we all have different genetics. For some of us we're lucky to have that butt whether skinny or not. Being a former fatty I was blessed to stay with most of it.
For skinny guys/flat back boys:
BULK, Any gym bunny knows bulking basically means gaining weight either while weight training (which is best in my opinion) or bulk just to gain if you want. This will help your body gain some fat which is needed to grow muscle. The gluteal muscles are the three muscles that make up the buttocks; the gluteus maximus, medius and minimus.So just like any other muscle, with training it grows. So obviously bulking + weight training = What? BIGGER BUTT! :)