Sento is simply a public bath. Here is a non-detailed description:
1) Enter & take off shoes in entrance way & put in shoe locker
2) Pay fee
3) Enter changing area for your gender
4) Take off your clothes & put in locker (or in basket on shelf)
5) Take your small towel, soap and shampoo into the bathing area (at more expensive places these are provided, at typical local places you bring your own).
6) Wash your body while sitting on a stool in front of a water facet. Rinse yourself with small bucket.
7) Go into baths and soak.
8) Do the reverse of (5), (4), (3) and (1)
Some places have a rest area have vending machines w/ beer & other drinks. Some nicer places have food like ramen, etc.
For a detailed description try searching on "japan sento etiquette" or something like that.