You can manually build a json. You'll need one for every block you have, and one for every item you have.
See Flan's Content Packs to see how they're built. You'll find them in assets>flansmod>blockstates, or model.
OR, you go by the Automatic building. Compile and build your pack; doesn't matter if the .jsons aren't built.
Then find your .jar, extract all of it's contents and move it into a folder.
Install that folder into your official minecraft folder, like how you would install a flan pack.
Run the game. This will automatically generate all the .json files you need.
Exit game and you can check the folder for them, where they'll be in the same directories as I said above.