Software gamification aims to leverage the user engagement with key features of software systems. Engagement is promoted by a conceptual gamification model that associates game elements (e.g., points) and rules (e.g., ranking policy) with features. It is quite common to gamify existing systems that were not originally designed with gamification in mind. Unfortunately, the development activities required to gamify a system are barely supported in practice. Gamifying an existing system requires: (i) the definition of a gamification model for guiding the incorporation of game elements and rules into an existing system; and (ii) the knowledge about key development activities to gamify this existing system. However, there are only a few models aimed to support the gamification of existing systems, and none is specific to software domains such as the prevention of mosquito-borne diseases. More critically, there is no systematic method aimed to guide the activities of gamifying existing systems. This Master s dissertation addressed the aforementioned limitations based on our experience with gamifying the VazaZika system. VazaZika is a system that encourages the prevention of mosquitoborne diseases such as Zika, Dengue, and Chikungunya. We introduce a gamification method that supports key activities during the gamification of existing systems, plus a gamification model composed of 12 game elements and 16 rules. Second, we evaluate our model with 20 users by means of ease of use, user interface, user fun, user motivation, and the potential for both constant system use and system dissemination. Our results are promising: (i) after some experience-based refinements, our method was successfully applied in the VazaZika gamification; (ii) we have identified 22 development activities that became challenging for developers to perform along the VazaZika gamification; these activities helped us to shape our method; and (iii) our gamification model has resulted in an easy-to-use system that is able to improve user engagement with critical healthcare-related features, such as the report of mosquito breeding sites. In summary, this dissertation contributed with additional guidance for supporting gamification of existing systems while shedding light on opportunities for future work.