1) Use the event to register your listener
2) During the prepare stage of the listener, parse the json files
3) During the accept stage populate your data structure(s) from the previously parsed json data
The subclasses of PreparableReloadListener provide many "frameworks" for common use cases.
I already linked a forge example,
or you can look at one of the many vanilla examples (recipes, loot tables, etc.)
or the similar processing of assets from resource packs (models, textures, etc.)
or you can search github for other mods doing something like what you are trying to do, e.g. https://github.com/search?l=Java&q=SimpleJsonResourceReloadListener&type=Code
Beyond that, your question lies outside the scope of this forum.
Feel free to ask more specifics questions where you have at least tried to implement something, rather than "tell me how to write my mod for me" type questions.