Here is a great sample discussion guide from Stanford’s HCI program.
This article provides a strong overview on usability testing and discussion guides.
The best way to learn about conducting a test is to watch. Watch Steve Krug conduct a sample usability test. His sample test starts at about 32:00 into the video, when he tests a participant in his workshop. Watch until about 42:00 while we see the participant struggling. This is a great example of the Hawthorne Effect, where the participant keeps plugging on and on to complete a task despite great difficulty.
The entire video is worth viewing – although much of the early video is a rehash of things you may have already read in Don’t Make Me Think.