If both you and her are mature enough to be writing "love notes" to each other go for it. If not, she'll think it's weird.
Describe her and what she does to you, emotionally.
Describe her eyes, her hair, the way she walks, the way she talks, DONT focus on her ass, or her tits.
Tell her you're the last thing you see before you fall asleep, when you close your eyes to see her, that when you're alone you want her with you, when you're sad you want her to cheer you up, when you're happy you want her to experience it with you.
/Shrug.