I want to know how I can create a completely custom texture atlas and how do I initialize it? As I understand in version 1.21 the atlas system is now fully described in json/mcmeta files and during client initialization an atlas is created from the listed texture files.

I need the texture atlas to implement my custom GUI. I tried to replicate the current Minecraft implementation by opening the client.jar source file however I am missing something.

Can you show me step-by-step how to realize the idea?