The second chapter of Steve Krug's new book - Rocket Surgery Made Easy - is a single page. He tells you to go to his website and watch him run an accompanied surf usability test.
The 25 minute video is right there on his website. Free to download. Whether or not you bought his book. You should though. It's very good.
Steve Krug demonstrates how to execute a usability testing session - 25 minute video
Free chapters of Steve Krug's Rocket Surgery book