1) You’re going to get naked. Completely. So the most essential item is a small towel to cover your privates. Everything else is optional. Most sento don’t provide soap and shampoo (although they sell them) so take these if you wish, along with a toothbrush and so forth.
2) Men and women bathe separately, so look for the characters 男 for male and 女 for female to avoid embarrassment. Once through, you’ll find a change room with lockers for your clothes and belongings. After undressing, all you should be wearing is that small towel and a locker bracelet.
3) You’ll find open shower cubicles or basins lining the walls. Sit at a stool facing the wall and use the small bucket provided to wash yourself with warm water (soap optional).
4) Enter a bath naked, or use your towel to tiptoe over if you are shy, but remember: the only thing that goes into a bath is your nude clean body.