As Mixed Reality (MR) technologies become more prevalent, it is important for researchers to design and prototype the kinds of user interface and user interactions that are most effective for end-user consumers. Creating these standards now will aid in technology development and adoption in MR overall. In the current climate of this domain, however, the interface elements and user interaction styles are unique to each hardware and software vendor and are generally proprietary in nature. This results in confusion for consumers. To explore the MR interface and interaction space, this research employed a series of standard user-centered design (UCD) methods to rapidly prototype 3D head-worn display (HWD) systems in the first responder domain. These methods were performed across a series of 13 experiments, resulting in an in-depth analysis of the most effective methods experienced herein and providing suggested paths forward for future researchers in 3D MR HWD systems. Lessons learned from each individual method and across all of the experiments are shared. Several characteristics are defined and described as they relate to each experiment, including interface, interaction, and cost. / Ph. D.