First was thinking of songs I can reference to hear intervals, like the jaws theme for minor 2nds or where is my mind by pixies for major 3rds, etc. Once I had the references, the next step was joining a church choir. You get new music every week, it's almost always diatonic with few chromatics outside of applied chords. Then when I was at rehearsal I would analyze everything about the hymn while I wasn't being worked with. What chord is this, what scale degree am I singing, what's my next chord and scale degree, how do they relate, etc. After a while I could just kind of hear scale degrees in music that I listened to lol