I am a first time car buyer, what do I need when I buy a used car from a private seller?

I was just approved for a pre-appoved loan so I would guess I am getting a blank check from them so I'd need that. I would need my license for ID is also a good guess. Also I think the buyer has to write something out on the back of a paper or something, which paper is this? the title? What else do I need? Do I buy the car and register it later or how does registration work?