Sure thing! As a quick disclaimer, Godot has a plugin that allows midi files to be read in as game data (made by arlez80), and this made the process of charting so much easier. I started out by making the music in my DAW (I use Cakewalk which is 100% free, but you could also use Ableton, Garage Band, FL Studio, etc.). All the instruments I used in the song are samples played using midi, which has a few advantages I'll talk about later.
Once I had the song covered, I created a new midi track for the game's song chart. In this track, each note on the piano roll would be interpreted by the game as an "input" for the player. This is where the trial and error begins, and you can move around the notes and expirement with what buttons you want to represent the different parts of your song. What I like about doing it this way is that you edit the game data in the same software as you edit the music, meaning you can reference the actual notes being played when you determine how you want to chart each section. This method also ensures that when you move the files into your game, your game data will be in sync with your music.
Once I had the game's chart where I wanted it, I did one last listen to the audio. Here I wanted to make sure that if the player would be switching to a different track in the song, that part would be audible/highlighted so that they don't get lost.
Once again I hope that this helps! Good luck on your journey as a game designer and future projects!