No one can tell you properly how to ask a girl out as there is no proper or correct way. You just have to be yourself, and if she doesn't like it, you wouldn't have had a good relationship anyhow. There is the standard advice already given, give eye contact. It gives the other person a good sense that you're sincere and honest and really are paying attention to them. Not to mention, if she's beautiful like you say she is, you'll get some pretty awesome chills. There is nothing like looking into a beautiful woman's eyes when you speak to her.
More or less, if you don't do it, you'll never know and will be wondering 'what if' for a very long time.
dm