I try to keep TestMod3 up to date with the latest Forge/Minecraft changes, so you can reference it to see how to adapt to recent changes or how to implement various features.
You can also search this forum to see if anyone has asked about the topic before.
Definitely check out these - https://wiki.mcjty.eu/modding/index.php/Main_Page
McJty is doing a great job explaining plenty of topics there and has links to other sites as well.
Create an instance of an anonymous class that extends StateMapperBase and implements StateMapperBase#getModelResourceLocation to return a dummy ModelResourceLocation (e.g. new ModelResourceLocation("minecraft:air")). You can then use StateMapperBase#getPropertyString to get a property string to use as the variant of a ModelResourceLocation.
You can see how I do this in my mod here and here.