Parents should tell their words about menstruation only after they've attained it. Even if they impart knowledge of this essential occurrence beforehand, there is very little explanation of the hows and whys of it. This one is for those at the brink of puberty. This also for the parents of those young ladies.

Menstruation in girls :-

The usual ages for vary from 8-9 years to nearly 15 years, with the mean age being 12. Imagine a young kid suddenly discovering that they are bleeding from there vaginas. Scary, right? Unless specifically educated regarding this, most will not even realise that it is coming from the vagina. One day she wakes up in a pool if blood and it seems to come from where she pees. And we just expect them to… grow up regarding this all of a sudden.
The first thing it won't be overdramatic of them to assume they're dying. Tell them that they're not. Neither are they ill. Some of their hormones have just decided to start their regular functioning. It's usually a good thing.
Secondly, tell them that too much pain is not normal. Do not encourage them to “live with it” Take them to the gynaecologist. Make the necessary diet and other changes. Take the prescription if needed. Too much bleeding is also not normal. Take them to the gynaec. Please.
Third, let them explore their choice of menstrual products. Sanitary napkins(even the type of napkin), tampons, menstrual cups, cloth pads- let it be their pick. In case of sanitary napkins, educate them about changing it often enough. (Every 3-4 hours is usually a safe deal.) Menstrual blood, once outside the body is prone to bacterial growth (it's full of nutrients- blood and tissue). This can lead to rashes and infections. In case of tampons, this is even more necessary. Toxic shock syndrome is famous for being caused by a tampon left in too long. The product you have used all your life may not be the most comfortable for someone else.
Fourth, while resting feels great, there are 2 problems- 1. It's difficult especially when you're still getting used to bleeding for minimum 3 days straight. Also, you're perpetually scared that you'll stain something adding to the discomfort. 2. Research has shown that exercise actually helps with cramps. Of course, it depends on the individual. More importantly, this exercise should be throughout the month not only the menses.
Fifth, remind your kid that this will be accompanied by other changes, all of which are normal. They are normal even if they happen earlier than their classmates. They are normal even if they don't happen at the same rate as their classmates. All of it just indicates that the sex hormones are starting to act. Again, usually, that's a good thing. (For those whose sex does not align with their gender, this stage might be especially difficult. Help them research their options so that by the time they are adults they can figure out what keeps them at peace.)
Finally, tell them they are not dirty. Menstrual blood is not dirty. It does not come from the urethra(as is the common misconception) and there is nothing to be embarrassed about. Tell them that the onset of their maturity is not shameful. Tell them that it is okay to still be a child, that you will still be there to answer questions and give out help when they seek it.
Make sure they can come to you for information and reassurance. That will require openness on your part and adequate knowledge.
Note :-

If I have left anything out, made undue assumptions, kindly point out. I have tried my best not to. Thank you.