Be honest with her! Is this a one night stand? Is it a relationship?
Do it *****
you don't tell her with words. It's all about body language and the moves you make. If you flat out ask her you might not get the response your looking for.
"Hi( insert name) -I want to **** you"
Buy her a large diamond ring and promise her you will care for and love her the rest of her life. Guarantee her that she is the first and last girl you will ever do this with for as long as she lives for better or worse, in sickness and in health.
If you have to ask, then she isnt the one for you.
Unless both of you are simply "fun" doing it, then just ask her right out.
Just ask her straight out. You're bound to offend lots of them, but the ones you don't offend.......