GitHub: forks, branches and PRs: important ➔ explain fork vs. clone!!!
strategies for keeping your fork up-to-date: your main and upstream's main, short-lived and long-lived topic branches
a more thorough and detailed explanation can be found on the Numpy Contributor's Guide. This guide can be adapted to your own needs, see gitwash.
make it clear that GitHub is just an option (git≠GitHub)
Scenarios
lone scientist working alone in the cellar without Internet (local git)
lone scientist uploading their software to the Internet in the hope it can be useful for other people (local git + one personal GitHub repo)
lone scientist sharing one software project with some other befriended lone scientist working in a different place (local git + o.p.G.r + permissions)
research group sharing software among members (local git + several GitHub repos + permissions + branches + [optional] PRs)
fully distributed software development using the most typical open source software workflows as used by numpy, scipy, sklearn, etc. (like above + we don't trust our contributors, i.e. work strictly with forks)