For me and my daughter (now 20) it was an ongoing thing from early on when she was a toddler. I was open and answered questions based on the info she asked about and what was age appropriate.
If your daughter is 9, she probably knows a lot more than you think she does because I am in the classroom a LOT and I overhear conversations with 1st graders and up about sex, bodies, etc.
I wanted her to know that sex, bodies, etc was not dirty and nothing to be ashamed of. Many moms are ashamed to talk about it, embarrassed and project bad vibes about sex and health being dirty, etc which end up damaging the child in the long run with misconstrued views of sex, health and our bodies.
There was an offer from Playtex to get a video and info for daughters and I ordered that for my daughter and I to watch. I did give her the book Care and Keeping of You which was ok to say the least. You could check the local library and see what resources they have as well.
My daughter started her period around age 12 and has used tampons since she started. They are just much easier and less messy.
The main thing is to build your relationship so she will talk to you feel comfortable enough to ask you anything. It sounds like you already have a good start with that! I know you want her to know the facts from you vs what she hears from friends. It is crazy what the kids say about sex, reproduction, std's, bodies, etc.
Best wishes to you, they grow up way too fast!