Hey @invisiblewasp, the #1 thing that aided my relative pitch skills was just trial and error – listening and paying attention when I hear a song playing somewhere, and trying to identify intervals that stuck out to me and the chord progression.
You won’t be able to do it perfectly at first, but make your best educated guesses as to what the chord may be or what that interval is in the melody. Imagine what frets you would need to play to play the basic melody on your banjo. Then, when you get home and are back with your instrument, verify and test how good your guesses were. You may be surprised at how much progress you can make without even holding your banjo!
Eventually, you’ll hear songs and begin to think “that part sounds just like…” and recall another time you figured out the same interval, chord, etc. in another song. As you go on, you get more and more familiar with how certain intervals or chord changes sound (some are just so recognizable!), and your training will simply build on itself.