Tutorial for calling C from pforth #220

I see this description in the pforth Reference documentation:

You can call 'C' from pForth by adding your own custom 'C' functions to a dispatch table, and then adding Forth words to the dictionary that call those functions. See the file "pfcustom.c" for more information.

But when I look at the code, I'm still drawing a blank on what to do. I was hoping you could add to the Tutorial a section on how to call a trivial custom C function, that perhaps takes a single argument, then the steps to build it. Please?