The emergence of Natural User Interface (NUI) two decades ago brought a lot of potential by supporting intuitive and multimodal interaction and leveraging human sensorimotor skills. Systematic prototyping and development of NUI, however, received much less attention, and legacy methods of the previous Graphical User Interface (GUI) were applied to develop NUIs. This has led to usability issues in many cases. In this paper we introduce a systematic lightweight approach to aid designers and developers in prototyping NUIs based on the context of interaction. We demonstrate the application of this approach through the development of a pen-based NUI.