How does one ask a girl out?

Try flirting first.
Asking a girl out, out of the blue will make her freak out or something.
If she doesn't shy away or ask weird around you, go for it.

Ask her out when you're hanging out or something casual. Ask her if she's single [even if you know she is]. That might give her an early warning that you're about to ask her out.
: )

Good luck.

well it all depends on the way you act around her if you haven't kissed her them maybe you should and then shell git the picture
if you have kissed her and she hasn't got the picture then just be straight up and be like would you go out with me IN PERSON.

Dude, I'm 38 and STILL do not know how to ask women out. With sexual harassment and women's independence, it always looks like women just do not want to be asked out at all. If you are a tall, very handsome guy, then you might be ok. If not, forget it. Ask a coworker or classmate out and you could be set up for harassment charges and never would have guessed she would do such a thing until it's over and done. Even women at a public place could get you in trouble if she was evil enough about it. Sure some women do appreciate a man approaching them but for the most part, 15 years of man-hating persecution has doe the damage.

If you feel confident, get to know her first before taking the chance. Try to just hang out first and learn about each other. Finally, make it very clear somehow that you are not trying to get in her pants.

Good luck, wish the best for you.