I'm still an amateur and always had problems with this, got some advice from an experienced jazz pianist I know. His suggestions were roughly to:
1. Start by transcribing the root note that you hear
2. Go back and see if each chord sounds major or minor - most of the time that will give you a major/minor third
3. Go back and play the 5th, and see if it sounds right - most of the time it'll be there
4. Don't worry about 7ths/9ths yet, they'll come, but give them a go
5. Once you've got a few chords, try and figure out the key, and that will help figure out others
So he was basically suggesting to transcribe by each note's function in a chord, starting with most -> least important. It still needs some music theory of course, but doesn't need you to listen to an entire complex chord at once.
1. Start by transcribing the root note that you hear
2. Go back and see if each chord sounds major or minor - most of the time that will give you a major/minor third
3. Go back and play the 5th, and see if it sounds right - most of the time it'll be there
4. Don't worry about 7ths/9ths yet, they'll come, but give them a go
5. Once you've got a few chords, try and figure out the key, and that will help figure out others
So he was basically suggesting to transcribe by each note's function in a chord, starting with most -> least important. It still needs some music theory of course, but doesn't need you to listen to an entire complex chord at once.