Why do you want to do this? Proportional is good
Well first find an exercise that only works your butt. Kneel down and put your palms on the ground. Then lift your leg up. It's best to do this with weight though. Go to the gym and find a machine that says hip extension
Ok now I'm going to teach you my building muscle method.
Warm up with light weight then add more weight until it's the most you can do for one repitition. Do as many sets as you can. The next workout session try to go up in weight atleast five pounds(2.5 lb. plates on each side). If you could do that go up five more pounds until you reach your max. It may sound crazy to be going up so much weight from one workout session to the next, but when you are a beginner the gains come quick.
When you plateau(a workout session that you can do the same amount of weight as the last session) stop working out for a week. This sounds weird, but it will make you stronger. A week from your last session go and try to add more weight than last time if you can do more take another week off(it's working!). When you plateau or get weaker with the take a week off plan go down in weight five pounds. Lowering the weight is good because the higher rep sets are for building muscle. When you plateau with that take as many weeks off as is useful and when you're back lower the weight five more pounds. Keep on taking weeks off and lowering the weight until you can do a hundred reps with the weight you're using. Higher reps than this are for endurance training. So after you plateau with hundred rep sets go down to one rep again. Work your way up to one hundred again. You'll find that before then your butt will be awesome.
As for the diet eat a lot of carbs. If you're worried about gaining fat do some sprints, ab work, and side bends. Don't do cardio because it's harder to build a big butt when you are running miles. Sprints are better for cutting fat anyways. Don't do sprints on the weeks you take off. When you take a week off don't do physical activity besides the necessary walking of course.
This works!