The impact of many interventions weakens during the scaling up process and 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 promote fidelity of implementation. The framework includes four steps that are typical of the development of new psychological and educational measures: conception, piloting and revision, implementation, and evaluation. We exemplify the use of the framework by describing the activities we carried out and the reasons for the choices we made in each stage to develop a tool to assess and promote fidelity in the scaling up of the Improving Working Memory Plus Arithmetic (IWM+A) intervention in a randomized controlled trial, involving 201 schools. In the scaling up process we used the train-the-trainer model. Twelve teacher leaders, who were trained by the programme developers, trained teaching assistants (the end users) who then 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.