Finding a sento, even if you don't read Japanese, isn't hard. Many neighborhoods have one; look for the common symbol for a sento, the Japanese character ゆ ("yu"), near the entrance.
Leave your shoes at the cubbyholes, or small (no charge) lockers, just inside the entrance. Pay the fee to the attendant, who is usually positioned next to the entrances to the baths, and she will give you a key to a larger locker inside.
Enter either the men's or women's section of the sento, according to your gender. Remove your clothes and put them in your locker, but take your grooming aids--soap, shampoo and so forth--with you into the bath area, as well as your towel.
It's very important to clean yourself before sitting in a bath. Hot and cold spigots (and often shower heads) along the walls of the sento allow you to do so, typically while sitting on a small stool, or standing if you prefer. The sento provides small plastic bowls you can fill with water and pour over yourself after soaping up, which is the Japanese style.
After you've cleaned yourself, and shaved if you like, then soak in the sento's hot water pool, cold water pool or whirlpool. Sometimes there is a sauna or steam room. Visitors can stay as long as they like, and leaving is merely a matter of getting dressed and returning the key to the attendant.
