Of late, there has been a rise in the use of touchscreens in many handheld devices, one in particular being the smartphone. This paper is centered on GUI design theory and explores the facets which make a GUI usable and flexible; further, this paper goes on to show the application of the GUI design guidelines and mobile usability studies to the design and development of an open source touchscreen smartphone GUI, which, in turn, will be implemented on an Embedded Linux development board which will serve as a hypothetical smartphone. The major outcomes of this study include the successful formulation and design of a hierarchical, touchscreen GUI suitable for a smartphone, as well as successful development and target-specific implementation of this GUI on an Embedded Linux, ARM-based platform. With the adoption of an open source philosophy, it has also been shown that the use of open source development tools can lead to a decreased cost of production.