MY DD was used to seeing blood, tampons and pads from being a toddler as she liked having a chat when I was on the loo. I explained in factual terms what was happening from when she was 7 or so. I also said that having periods showed you that you would be able to have babies if you wanted them.
My mum never explained anything to me. The consequence was that when my breasts started to develop, I thought I had breast cancer because I had overheard my mum talking over the fence with a neighbour whose elderly mother had the disease. It had started with 'lumps in her chest'. As a result, I was determined my DD would not be terrified for months as I had been.
However, I had done a less good job than I had hoped. DD had a phase of being obsessed about how big her breasts would be ('Will they be as big as yours?' 'I don't know. You'll have to wait and see what instructions your genes have been given about them.') Then DD 'I want to have breasts but I don't want to have to wear one of those eye-patch things on my fanny.' Me: 'Too bad. You can't have one without the other.' Finally, and out of left field in Year 6 when she had had what she called 'sexy education', 'If you are in pain when you're having a baby, it feels nice if you touch your clitoris.' Think very quickly, I thought. So I said: 'There are lots of times when touching your clitoris feels nice, not just when you're having a baby.' Perhaps the NHS should encourage this method of pain relief to cut budgets for pain relief....
When DD's period started, at 11, we went out to a local tea shop to celebrate and I talked to her about our female ancestors back to 1860, the furthest back I've managed to research.
From all that, my advice would be normalise periods and recognise that you are not the only source of information.
Edited for typos