After installing the mod and running your game with it once you will notice 2 things:
- You have a new button in your main menu, that is a refresh button that allows you to reload the json files without restarting your whole game. You can also do that by holding ctrl + r.
- In the Custom Main Menu config folder will be a file called mainmenu.json, that file is automatically generated when cmm can't find a json file for the mainmenu. By default it contains a exact recreation of the vanilla menu in json. If you wanna change the main menu, modify that file.
Generally you can learn quite a lot about how to modify stuff by just looking at the default json file.