How do I make custom templates?

Closest post here I was able to find was this one.

https://groups.google.com/g/pandoc-discuss/c/CLUVrVkJlls/m/-IQDg3pGCAAJ

However it and the page it links to doesn't actually tell you how.

The page it links to gives you an example of the code needed for it...
without telling you anything about how to use it with Pandoc.