This paper presents a simple method for the initial design and validation of graphical user interfaces by group exercises based on brainstorming and storyboarding with paper prototypes. It requires no technical skill on behalf of the participants, and may thus be used with end users. The method helps to initiate a group process and motivate them-by getting these people involved early, the acceptance of the overall development effort may be improved. The approach also proposes a syntactic variant of UML state transition diagrams as a User Interface Design Language (UIDL).