Adding a Song
To add song files into your mod, create a folder named songs in the main mod directory (the one with your _polymod_meta.json in it).
Then create a folder with the name of your song in ALL LOWERCASE (can have special characters) inside of the songs folder.
After that, just add an Inst.ogg (and most of the time, a Voices.ogg as well) to the [SONG NAME] folder that you just created.
To add a chart for the song into the game, create a folder named data in the main mod directory.
Then create another folder inside of the data folder named song data, then inside that folder, create a folder that has the same name as your [SONG NAME] folder.
It should look something like this:
Then inside that folder? Just put the .json charts of your song.
You don't need all difficulties, but any difficulty that isn't normal mode has an -[insert lowercase difficulty name here] at the end of it's name before the .json.