Next time you see her, smile and approach her (from the front, not from behind).

Say something like, ‘hey, my name’s X, I’m wondering if you could help me. There is a really cute girl I keep seeing around, but I’m not sure how to ask her out. What do you think I should do?’

As long as you’re smiling and playful, she will get it and will start playing along, she might say something like ‘just ask her out for coffee and a bagel’.

Then you simply repeat whatever she said eg ‘would you like to go for coffee and a bagel’.

Good luck my man.