Technology, software systems and human-computer interaction paradigms are evolving. Traditional usability heuristics do not cover all aspects of user-system interactions. Many sets of heuristics have been proposed, with the aim of evaluating specific application domains and their specific usability-related features. In addition, several sets of heuristics are used to evaluate aspects other than usability that are related to the user experience (UX). However, most authors use an informal process to develop usability/UX heuristics; there is no dear protocol for heuristic validation. This can result in sets of usability/UX heuristics that are difficult to understand or use; moreover, the resulting sets of heuristics may not be effective or efficient evaluation tools. This article presents a formal methodology for developing usability/user experience heuristics. The methodology was applied in practice in several case studies; it was also validated through expert opinions.