Templates for ResumeFodder are Microsoft Word 2003 XML files, with text values inserted by the Go template engine.
Although a bit arcane, the Word 2003 XML format is actually quite thoroughly documented:
However, you can probably figure out most of what you need by simply reviewing the existing template files in the ResumeFodder-templates git repository.
If you have no experience (or at least interest) in Go, then creating new templates for ResumeFodder may be a daunting task. Otherwise, the process is fairly straightforward:
data/data.gosource code file.
templatessubdirectory of your local ResumeFodder-cli installation. Optionally, make a fork of ResumeFodder-templates and issue a merge request or pull request.