Hello Pierre, nice idea! I agree with Ricardo's suggestions.
You could have a look at the slides of a talk I gave at GHM 2013 in the "maintenance" git repo, inside the talks/ghm-2013/andreas directory. Some things are outdated (we did not have the macros to manipulate build phases yet). There I start actually from an even more basic view point, since the target audience was people not knowing Scheme at all: first, a few basic notions of Scheme (quoting and unqoting is needed all the time); then how to modify an existing package without inputs using gnu-build-system; then how to handle inputs; and so on. Andreas
