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.