First of all, let me put it as plain as simple:
Don't think about it, do it.
I can't stress the importance of not thinking, stutter, look like an idiot, if you're the type to not smooth talk naturally, she'll either find it cute or turn you down, either way if you're not a natural at it, it's going to show after a few days of a relationship and all excessive thinking is going to do is make you look like a coward cause you're taking ages to tell her, even if it's obvious from your behaviour already. She also might hook up with someone else by the time you decide to tell her.
And just to persuade you to the wisdom of my words, here's a very good motto I try to live such moments by:
"10 years from now, you'll regret the things you haven't done, FAR more than the things you have done."
edit:
Also. Learn to deal with rejections. Don't make a big deal about them, if you take forever to tell a girl how you feel about her, you're only going to fuel your affections for her before you tell her and it's only going to make it hurt more if she says no.
If you figure you like someone, come out with it and say it. She says no, you did it early and the shit feeling won't last longer than a day. Move onto another girl until you find someone with mutual feelings and you'll be happier for it in the end
