I sometimes find musical gems that I'd like to jam with but upon discovering that sheet music for them don't exist, I take on a transcribing challenge because it's such a good exercise. I don't have a degree in music, but I have a strong understanding of scales, chord degrees, harmony, and the range of extended chords that you can find based on the the scale. I typically start out by listening and writing down the lyrics, then playing along with a piano until I can mark chords above the text. Only when I'm confident with that, I take it to a sheet music software where I have to also figure out the rhythms, durations, and expression. The most common type of music I probably transcribe tends to be a 70s-80s recording of black music; The recording technology isn't always the clearest, and you hardly ever hear pure minors or majors that would only consist of three notes.
Many times, I've been playing along and writing down those chords and it sounds okay, but later, I lose confidence in it, because on its own some places don't sound quite right. In confusion, I start meddling with the lines that don't sound right, and I may tweak it even more off, just to return back to the previous version where I can't quite pinpoint what's wrong. I suspect that with the very complex harmonies typical to black music, the same chord enharmonically either contains multiple individual chords, or ones that are very close, so what you hear in it isn't as obvious as if you were listening to something of the level of "Twinkle twinkle little star". The warm fingerprint of a tape recording with some vinyl crackle on top also don't really help with the clarity.
I'm suspecting my technique, feeling frustrated and stuck. What do you think is the best order of steps to take on getting this right, what should the workflow really be? How do you deal with the complex harmonies where there's so many options?
Most chords that I write down that end up to be wrong sound like they could be right, but it isn't the one in the recording, so it's just a matter of what kind of hues the composer has chosen. A bit like: sure you can jazz up even Twinkle twinkle little star, and with completely new chords, it can still sound musically correct, but that little star is going to be in a really different mood. It's all about context: if you had the usual Twinkle Twinkle little star with the plainest possible chords but substituted one chord with some really exotic and colorful one, it will sound alien. While a chord might technically fit, it still has to fit the context and follow the overall style.