Personas are fictional but realistic representations of user groups of a product or service that focus on their goals, needs, behavior, and other characteristics that may impact the design of the product. Using personas in the early phases of the design process can help identify product features, and demonstrate how these features address user needs and challenges. Despite being used in research for over a decade, there is a lack of standardized methodology on the development of personas and how they are used in the design process. The objective of this paper is to develop a systematic approach to the persona development process and show its effectiveness through a case study where we followed the proposed methodology in the early design phase of a diabetes management smartphone application (app) targeted at older and younger users. The study was conducted in United States from January to June 2020. The findings of our study provided an empirical examination of the effectiveness and efficiency of the design process using personas.