I would be amazed if your daughter hasn't heard of menstruation by now. You might not have mentioned it to her, but I'm sure the girls at school have talked about it.
I've never figured out how kids DON'T know about menstruation. Their mothers must have their feminine supply products beamed directly into privte bathrooms from space, and they must not carry supplies in their purses. How else would a kid never see a pad or tampon?
I don't have any practical advice for you because my daughter has known about periods practically forever. Of course, as a little kid she didn't understand the whole reproductive cycle, but she knew that grown-up ladies get their periods every month.
Cute story: When my daughter was a toddler I used washable cotton pads, which I kept in my underwear drawer. One day I heard her rummaging through my drawer, and by the time I got there, she had put a pad into a pair of my panties, pulled them on over her diaper, and was walking around, holding them up by the sides so they wouldn't fall down!
I was around nine when my mother told me about periods. I had already heard of them at school. I guess I DO have practical advice -- think about how your daughter might misinterperet what you say or what you give her to read, because the booklet my mom gave me made it sound like women bleed continually for years and years without end. I knew THAT wasn't right, so I figured the rest of the book was wrong, too. (It wasn't.)
P.S. I'm adding this on: Among my friends it's popular to have a "Period Party" when a girl gets her period for the first time. Everyone and everything is red, the attendees who already have their periods give advice and share stories, and the buffet table is loaded with chocolate!