1st, place your front foot like you're going to do a heelflip. i know its sounds weird but you're foot will tend to slide off when you're trying to turn fast, so keeping your front foot toes hanging off a little makes that adjustment for you.
2nd, make sure you get your shoulders turning FS before you pop. i see this happen all the time and i can't stress this enough. if you don't turn theres no way to land the trick.
3rd, pop as high as you can but spinning fast is more important, once you pop and it gets past 180, use your front foot and scoop/push it down and around the other 180.
its kind of hard to learn them on flat, so try doing it off something small like a oversized curb or ledge/drop so you get a little more time to turn the entire 360.
if you learn them off a little drop, you'll have plenty of time to get it around, and you'll also get a better feel for the trick(the spinning, landing, foot placement). after you get use to landing them off a drop, then you can them take it to flat and it'll make things much easier.