Review: Paper Prototyping: The Fast and Easy Way to Design and Refine User Interfaces
If you want to learn about paper prototyping from a renowned practitioner then I highly recommend Paper Prototyping by Carolyn Snyder. Snyder advocates paper prototyping because it’s easy to design (requires minimal drawing skills), cheap to create (needs only paper and markers), and offers and opportunity for developers and users to evaluate design concepts. If you wonder where the beginning of the design process starts, it begins when great minds meet and brainstorm ideas, and drawing is a natural approach to illustrate them.
Dick, David J. Usability Interface (2004). Resources>Reviews>Workflow