Many interventions lose their impact during the scaling up process; low fidelity of implementation (i.e., delivering an intervention but not as it was intended) may explain why. In this paper we introduce, discuss, and exemplify the use of a framework for developing fidelity tools that aim to measure and to promote fidelity of implementation. The framework includes four stages that are typical of the development of new psychological and educational measures: conception, piloting and revising, implementation, and evaluation. In order to exemplify the use of this framework, we describe the activities carried out in each stage to develop a tool to assess and promote fidelity in the scaling up of the intervention Improving Working Memory Plus Arithmetic (IWM+A). Scaling up was achieved through a train-the-trainer model in a large RCT involving 201 schools. Teacher leaders (N=12), trained by the program developers, trained teaching assistants (the end users), who delivered the intervention to children in the intervention schools (N=100). We describe the intervention, the training model, and the process of developing, piloting, implementing, and evaluating the tool. We also discuss implications that derive from the framework.