The place to start is, I think, the FOND resource definition in the Rez
header of the Universal Interfaces.
These files have extension ".r".
(I never used a higher version than 3.4.1)
Then you can write a C program to fill such a structure.
There has been also a useful program, I think it was called FOND Detective,
that can teach you useful things about your fonts, the sort of information
that you want to turn into a FOND.
This program runs on old computers, I think OS 8.1 will do.