Choose a phase whose tasks you are about to do. Writing the course is how you learn it, so the person doing the tasks writes it.
Copy _template/ to romulus/pN-name/, fill in the modules and questions, and add one line to assets/catalog.js.
Open a pull request. The other person takes the quiz before approving it: if they can't pass it, the course isn't clear yet.
The briefs above come from roadmap.json in the robot repo. After changing the roadmap, run node scripts/sync-roadmap.mjs here. Details in CONTRIBUTING.md.