Objective: This study describes the step-by-step development of the IMove30+ programme and outlines lessons derived from the authors' experience using an intervention mapping protocol (IMP)-based programme design. The programme was designed to increase the moderate to vigorous physical activity (PA) level at school among Lebanese children, aged 10-12 years. Design: Participatory cross-sectional design including group and individual interviews. Setting: Sidon district of Lebanon. Method: The programme was co-designed with a planning committee and included a local leader in school health, school staff including nurses and members of the target population (schoolchildren). The programme was developed using the six steps of the IMP: elaborating a logic model of the problem, formulating programme objectives, choosing the theoretical methods (i.e. the theory-based techniques used to influence a change objective) and practical applications (i.e. the applied strategies based on those theoretical methods), designing the programme, planning programme implementation, and planning the evaluation. Participants' involvement in the programme's activities was entirely voluntary. Results: Implemented by teachers and school nurses, this 14-week school-based programme was designed to provide an additional 30 minutes of school-based PA per day through structural environmental change, educational activities, a PA-monitoring system and PA events at school and in the classroom as well as during recess. Conclusion: IMP enabled the rigorous and systematic development of the programme to improve children's PA level. The programme description and the lessons learned can facilitate the replication and the scaling up of the programme in other settings.