Guide written by EvilSnail0fDestruktion
Start with the Original Class level {x} progression from base game in front of you. This will not be in your mod.
Make a copy of the Original Class level {x} progression from base game with a new uuid and set to multiclass (it should have all base game subclasses and no modded ones).
Make another copy of Original Class level {x} progression from base game with a new uuid and your modded subclass(es) added (it should have no base game subclasses).
Make a third copy of Original Class level {x} progression from base game with a new uuid and set to multiclass and modded subclass added (it should have no base game subclasses).
Contine making the subclass like normal from here!