@millert1409 Thanks!
I don't have an exact method for this. In fact, I feel much more comfortable with no classic harmonies.
In the case of "Caronte y Psique", in general terms, it flows between classic tonal parts (the cadences) and other parts where the tonal center doesn't exist at all. Let's say it begins in a chord (Gm) and it ends the phrase in that chord, but in the middle functional progressions aren't used. There are many changes to transient tonal centers going up and down by mediant relationships (most often chromatic), that is to say, by thirds (mayor or minor). It happens in the picture above: Gm - Bbm - Dbm - Fbm..... rising up by minor thirds (that's why I use Fbm and all those double flats; yes I could have used Em instead, but then the mediant relationship is not represented). When I work in this background, many times the chords change just because they flow together well. I think this is how impressionism works, among other things.
If you are interested here you have the score with some notes, and the harmony notated.
Caronte y Psique cifrado notas.pdf