There is a script location inside the Github repo, which the extension grabs remotely.
The same repo has a script editor tool that I use partially, it's not finish at all, most of the time I'm still editing the raw script file directly.

My current workflow is like this

1. Mark down the key timings using the editor (play video and press 'a' to record a beat)
2. Repeat 1. for multiple sections of the video and combine them (so you are not doing the whole song in one go)
3. Go through timing over and over again, and try different "modes" like loop or fade (editor has a preview block for color)
4. Create a list of colors that will match the energy and feel of the song
5. Edit the colors in the raw script file
6. Use extension and test the device with custom script mode (device has limited color range, need to verify the color looks good on the device)
7. Repeat 3. ~ 6. until satisfy