Well, I'm a pretty skilled writer, if I do say so myself. And I'm a soppy romantic. However, I can't tell you what to put in the letter, not knowing either you nor the girl in question. So if I can't tell you, then I doubt many of the other members of this site will be able to (not to be too egotistical

).
Seriously, just write from the heart. If you aren't the romantic sort then there's no point doing a love letter, so you're better off talking to her directly and just asking her out sometime. If you are a romantic sort, then you shouldn't need any help here.
Failing that, just PM BonsaiK...