I want to create a template for an operating system that does not exist in the community, so please tell me the correct way to do it.
What is the OS?
What services do you want to provide in the new template?
It’s simple to provide HVM templates, even packaged.
Somewhat more difficult to provide templates to provide AppVMs.
More difficult again to provide inter qube services like qvm-copy/move.
Trivially easy to provide fully featured templates that are
derivatives of existing templates, like Debian, Fedora or Arch.
“The correct way” will depend on what you want. Tell us.
I never presume to speak for the Qubes team.
When I comment in the Forum I speak for myself.
I’m not sure whether the instructions you linked are still applicable, as the docs for Qubes builder v2 doesn’t appear to mention anything about the Builder Plugins API, and the instructions you linked were last edited before Qubes builder v2 released.