I have been to an onsen on a number of occasions, and it is a lovely experience. Particularly when you come from skiing a mountain, and at the foor od the slopes is a ski-up traditional spa/steam room!
I'd suggest you might want to go with someone who knows the procedures though. As th staff likely won't speak English, unless it is pitched at a western market (not that I've seen, but they might exist at the high end these days?).
The places we went to, you went in, undressed, and then with no more than a long flannel, for your modesty, went to a row of 'positions' where you had a mirror in front of you, a hand held shower hose and soap. Here you scrub yourself spotless, every square inch, while sitting on a little stool. This area usually was adjacent to the hot plunge pools, with rows of hot and cold showers nearby... all one big open area. So, go in, change, clean up, then go and sit in one of the hot pools. Then it is traditional to go from a cold shower to hot a few times, before going back into a hot pool.
Bloody lovely!