That's really sweet of you. Problem is, you don't want to use other peoples words to pass on through generations do you?? I will give you ideas to think about...
During the day, write down funny moments your daughter does and how they make you feel. Even if your writing is simple, it's still coming from you and your heart. Not everyone writes beautifully but everyone can write a letter that someone else reading can say, "yep, that's A.". Make sense?
When your daughter is sleeping, pick up a pen and paper and describe how she looks. Is she peaceful, does she suck in her bottom lip, does she move a lot etc....
When you are feeding her, write down her favorite foods and how clean or messy she gets or maybe how you don't like her to be messy so you keep a wet cloth nearby to wipe the smallest of food off her cheek.
When she's letting our her full personality, who does she remind you of? Is there a funny story about that person you can write down to prove it?
When taking walks, is she alert and points at everything, screaming out everything she sees or is she laid back and enjoys the ride?
The words will come out of you, just make small notes throughout the day as you think of your baby girl and before you know it, you'll have a beautiful letter straight from you, not a random stranger on Mamapedia:)
GL!