The hints on that mentioned homepage are really recommendable. Unfortunately I have never been to Japan. But I have been to Iceland, and they have also those wonderful hot baths, the so-called hot pots in Iceland. And they can be REALLY hot. If you go into the wrong water, you will be cooked by about 80 degrees. But the normal hot pots for bathing are also hot and you should go in very slowly and warm you up step by step.
But they were sooooo wonderful, and I can imagine that an onsen is also so wonderful!
