Just say, "hey, you want to go out sometime? And by that I mean, I am asking you out on a date, yes no, maybe"
If she said no, just laugh and say "oh, you suck, man, alright, take it easy. FYI I only ask a girl out once, so you already lost your chance." and leave it at that.
Although sometime reverse psychology work with some very hot girls (Don't know why, don't care about why) So when you say "FYI I only ask a girl out once, so you already lost your chance." there is a small chance she might grow some interest in you and that might be enough for you to make her your girl.