Training teachers in evidence-based practice is a societal challenge. We conducted practical action research to investigate the impact of a professional development programme (the aim of which is to train teachers in explicit instruction) established according to the principles of effective professional development on one teacher's practices. A holistic case study was conducted with one teacher randomly selected among a group of volunteers. An original methodology was developed to measure the fidelity of the teacher's practices to the different teaching practices and stages of explicit instruction. The teacher's practices were filmed on four occasions and analysed using an observation grid based on the literature on explicit instruction. The advice and intentions to act expressed during coaching sessions were listed. By comparing them with the teacher's practices, it was possible to identify whether or not they had been implemented. The results indicate that after the first coaching session, the teacher was able to implement the stages of explicit instruction. During the following observations, her implementation of explicit instruction was even more precise. However, she did not implement specific checking for understanding. These results support the interest of directive coaching to support the implementation of explicit instruction.