Start with little things now to work up to the big stuff, I'd been bringing up things with my own dd since she was 2 and realized her friends privates were not the same as hers, then onto where (very generalized) babies come from when I was pregnant with her brother, and more detailed later. She was present at every ob appointment and at his birth (she was 5 and yes really right there next to the midwife all gowned up and cut the cord) Then we worked up to puberty changes a little at a time as necessary but also discussing the things to come. In 5 th grade this allowed her to feel comfortable enough to ask her male teacher permission to go "speak with female teacher next door" when my dd told her she had just started and did female teacher have a pad she could use female teacher said sure discretely handed it to her and when my dd asked for a pass to go call me the teacher asked if everything was okay dd said " Oh yes we discussed how it all works I just want her to know because I'm too excited to wait to tell her I finally have my period" Female teacher "Wait this is your first?" dd "yes" female teacher "I've never met someone so young so calm about it." dd "Then I guess my mom did a good job huh?"
and she called me excited that she was "growing up" we went on a special shopping trip to get her things she would prefer to use, a box of chocolates and her first pair of high heels. Get a book, give it too her there are tons that are good tell her you'd like her to read it and why and that you guys can talk about what she reads ask her questions about the book how it makes her feel etc. My mom did all this with me and I with dd. We never made it taboo and started early. It's still weird regardless but it will make things a lot easier later.