Sure, I'll bite. I can't say my experiences are 100% representative of the norm--but then again from what I understand it's different for everybody.
For me it comes progressively. The biggest tell-tale sign is that about a week beforehand my chesticles (favorite new term--not sure who around here coined it) like to get a bit sore around the nipples, and then sore throughout. The actual breast tissue--mammary glands, not fat--begins to ache and swell. (I hate that part; it makes the things big enough that my usual binding method goes from adequate to insufficient--the damn things bounce.) I believe estrogen is at its highest at this time as well, which I suppose explains this effect.
The ache in my chest continues until bleeding starts; by the end of the bleeding, my chest is back to its normal size. The first signal that bleeding is going to start is that, on the same day, I will begin to get chills accompanied by goosebumps and a cold sweat. It's a whole-body thing, and very much like a flu symptom. I may or may not get off-and-on cramps before actual bleeding starts. The first and second days are the worst, with my body temperature constantly fluctuating. Extreme cold is felt as my abdomen cramps and then peaks in pain, subsiding as a hot flash comes on. The entire cycle takes ~2 minutes and repeats. It is relentless and pain meds have no effect--though Tylenol Extra Strength used to work wonders when I first started. This is a pattern I have noticed since I first began, but it has gotten much worse in recent years to the point of not being able to function. What began as fairly mild chills/hot flashes and a little cramping has... amped up considerably; I hope your daughter is one of those with milder symptoms.
Another intriguing change is that the cramps are beginning to no longer 'cycle' between pain & no pain--they maintain a sharp ache throughout and only the intensity of the pain changes from ache to stab. Before, the cramping was a cyclic dull ache and not too bad. So yes, this is very much an entire-body thing; I can't say too much about bloating, though the area on my abdomen directly in front of the uterus does get swollen and hard during this time. Other symptoms include frequent urination & defecation (sorry, TMI, but the uterus really can wreak havoc with the surrounding organs), a general ache in my hip joints.
The bleeding is constant and heavy until the third day; things start to taper off may stop completely, and the pain is gone. It may stay like this for a day or two, but then the pain will return full-force, heavy bleeding will return, and this will stay for about a day or two until it stops again (tapers off slowly), hopefully for another 3 weeks--but the cycle is very irregular for me & always has been, so I can't really say. I once stopped completely for 8 months until I finally gave in, went to a gyno concerned about my bone health, and she prescribed me estrogen
The above is just my experience and may not at all represent what your daughter will have. Like I said, I think I may be on the nastier end of things. I can't really give you any tried-and-true advice on what to say; my parents really didn't have 'the talk' with me and left it up to the school to educate me (they have sex/puberty-ed in the 5th grade). That being said, I was in horror and disbelief when this actually happened and kept it a secret from my mother for 7 years simply because of a video they showed that highlighted a scene between mother & daughter, where the mother looked to her daughter with great emotion and said "You're becoming a WOMAN now!" *Really* didn't want to hear my mother saying the same thing. Anyways. Maybe just keep it to the facts of: you will begin to menstruate, you may experience cramping but Tylenol & heating pads (I really recommend getting one, they're awesome) help with this... don't make it sound frightening (it is a natural bodily function after all) and just kind of keep the lines of communication open with her. Here's a site that may be helpful:
http://www.iwannaknow.org/puberty/girls.html. Which reminds me of one more thing regarding tampons: a life-threatening, though rare, bacterial infection called Toxic Shock Syndrome can develop if they are not changed frequently. Just something to be aware of. Pads don't cause this, but the downside to those things is that they are bulky and don't smell too good after a while. I believe most girls prefer tampons & they are safe if changed as per manufacturer's instructions (seriously, every package comes with a warning now).
I think that covers it; I hope I was (mostly) clear.
<i>Note: Sorry, Sarah, I saw Kiera's post and somehow started thinking you were the one needing to explain things to a daughter. Anyways, from my experience when women talk about their periods, I dunno, they kind of just share what theirs is like & use it as a time to sort of air out their complaints with it (general consensus: it's sucky, but at least it means they can have kids). Don't know if you want to make up an imaginary cycle to talk about or not... It really might be sufficient to say that you are not able to menstruate--it does happen sometimes. If they ask why, well, you're on your own on how you want to answer, though you could just say "I'd rather not talk about it"--because, really, I think some women would be pretty upset about not having a cycle, especially if they are of child-bearing age.</i>
