Generally, you show up, remove your clothes in a changing room and leave them in a locker, put on a pestemel (traditional towel, thinner but about the same size as a standard beach towel), and enter the hamam. Many women leave their bottom underwear on or bring a bikini bottom, but I've never seen anyone anything on top. I'm not sure about protocol for men. Some hamams (like Cemberlitas) provide you with bikini bottom-like underwear so you don't get your own wet.
There's usually someone to lead you in and they will put your pestemel down on a hot marble slab and direct you to lie on it. They may pour warm water on you. You stay there for about 15 minutes just heating up - the scrup if more effective if your skin is moisturized. Then a person (usually same sex) comes to scrub you with a rough exfoliating washcloth head to toe. After the first scrub he or she will rinse you off, then do another scrub with a soapy foam that helps moisturize your skin. They might wash your hair, but this isn't the norm.
Afterwards, you're usually welcome to sit in the hamam as long as you want. There are a number of sinks around the hamam where you can fill a bowl up with warm ir cool whater to rinse off or cool down. Most hammams have a shower to rinse off on your way out and and/or wash your hair. Some have a seating area where you can hang around in your pestemel and drink coffee/juice/water. Some nicer hamams will have hair dryers as well.
Also check out: What are the best 10 hamams to visit in Istanbul?
