we will call your fantasy grounds data folder \fantasy grounds
in there you have a bunch of other folders like \tokens, \rulesets, \modules, \extensions and more
inside these you will find various files often with specific file extensions like .mod for modules, .pak for rulesets and .ext for extensions
these are actually zip files in a specific layout and with the correct extension for the data type
you can do it without the zip file too
so for your extension you want to create a folder \fantasy grounds\extensions\myextension
and your files live in there starting with extension.xml
you can zip it up but generally you dont until you have finished building it - its much quicker to keep hacking away at the unpacked files rather than unpacking, edit, repack, test, repeat