Achieving the competence to reflect on the degree of suitability of textbooks is not easy, so it is necessary to train prospective professionals in the knowledge and use of analytical tools to guide them in critically analyzing these curricular materials. In response to this research problem, this paper describes and analyses the implementation of a training intervention that aims to promote reflective competence in prospective Costa Rican teachers through the analysis of textbook lessons. The content of proportionality is chosen because of its central in the curriculum. The training activity was motivating for the participants and helped them to progress in the competence of lesson analysis. However, it is noted that the design needs to be improved in terms of time dedication, as well as attention to the necessary didactical-mathematical background knowledge on proportionality to achieve better results.