This page talks about the basics of making a mod for Nova Engine Godot (VERY WIP LOL)

You need your song's json, inst and vocals

1. Add a folder titled your song's name in the "assets/songs" folder

2. Put your song's Inst, Voices and Json in your song's folder

3. Make sure you name your chart's just the difficulty. (Ex: Not "song-hard.json", you would name it "hard.json" or whatever difficulty it is)

4. In the Freeplay menu, you select "freplaySongList.tres" in the Godot editor

5. To add a song, you click "Add Element", "", "New FreeplaySong". Then click on it so see the information that you can edit

6. You can configure the individual values to your liking.

7. Open the game (by either pressing F5 or clicking the arrow in the top right) and your song is there!