That is and very most likely will remain undocumented. Qubes-Whonix minimal templates aren’t offered, unsupported and neither planned.
Qubes builder is:
rather complicated to use,
last time I checked sparsely documented,
downloads package from the Qubes (and Qubes-Whonix) binary repository instead of building them during the template build process. What it means to build “from source” in context of a Linux distribution isn’t well defined. More on this point: Builds from Source Code versus Builds including Binary Packages
These issues are specific to Qubes and unspecific to Whonix.
Prerequisite skills:
How to build other templates using Qubes builder.
How to build Qubes-Whonix using Qubes builder.
How to set an environment or build config variable (that becomes an environment variable) in Qubes builder.
Once you know all of the prerequisite skills (which are unrelated to Whonix support), let me know.
Then the Qubes-Whonix specific part is by comparison rather simple. Hints: