The great advantages of open source code-based projects like the case of open source firmware (OSF) are flexibility and freedom of customization. However, the difficulty inherent to their development process, which can be seen as a software composition issue, is the lack of structured approach and a teachable methodology for efficiently tackling such project. In this paper, we propose a 5-step pedagogical OSF’s customization approach, coupled with an agile development process to guide the learner and ease his comprehension. We experience this approach to prototype WiAFirm, an OpenWRT-based firmware for operating IEEE 802.11x enabled WAP. Two groups of 04 students each were involved. After 2 months of experimentation, the group that applied the approach was able to integrate into the core OpenWRT a custom WifiDog captive portal feature as a built-in package; while the other group had barely understood the goal of customizing an OSF.