How can you get a big butt quickly if yours is flat and you're not genetically inclined to have a big one?

If you're not genetically predisposed to developing a strong, round butt, glute implants might give you the look you want but can't achieve naturally. Note that if you are not inclined genetically to have a large, round butt, there is no short-cut to achieving one unless you have surgery. Run up steep hills regularly, 3 times per week. Do squats, sometimes called deep knee bends, without weight at first, using just your body weight. You can then add resistance, if you want, by holding dumbells in your hands. Squats are great for building up your quads and glutes. Skating -- roller or ice -- is a great exercsie for building muscular, round butts.
flirty girl fitness!

Running up and down sand dunes or hills, or regular climbing of some kind.

Interval track work on regular basis would also help.