Well, if you're the kind of person who gets quite nervous and finds it hard to speak, you could write a letter and give it to her. That way, whatever you want her to know is written down, and you don't have to worry about not getting the right words out or etc., and then just ask her if she has any questions or something.
I can't really give any advice on actually telling someone without a letter, because I did the letter thing. But I think just take a nice, deep breath and start off with a small sentence, so it's not a lot of words you have to think over and over again to get it right. Then just take it from there in whatever way you think is best.
Good luck, hope it goes really well