The teaching of computer programming will soon become compulsory in the gymnasiums of the canton of Fribourg. To this end, we are conducting a project to co-design a serious game and an associated pedagogical scenario. This project is carried out according to a design-based research (DBR) involving the collaboration of an interdisciplinary group. In this article, which is based on and extends a recent communication, we define the DBR methodology, then analyse the co-design process with the successes and difficulties we encountered. We discuss the factors which facilitated collaboration and ownership of the research by different members of the group.