Ear training improves fastest when the task is small and feedback is immediate. Fifteen focused minutes every day are more useful than a long session once in a while.
Minutes 1–3: find the tonic
Listen to the end of a phrase, sing the note that feels settled, then find it on your instrument. Establish a likely center before hunting for chords.
Minutes 4–6: follow the bass
Focus on low notes at chord changes. The bass does not always play the root, but it narrows the possibilities dramatically.
Minutes 7–10: major or minor
On one sustained chord, test both major and minor triads above the same root. Pay special attention to the third.
Minutes 11–13: hear degrees
Once you know the key, think I, IV, V, vi instead of only chord letters. Functional hearing transfers to any key.
Minutes 14–15: verify
Check with an instrument or a reliable chord chart. Write down why an answer was wrong. Automatic analysis can be a useful second opinion when you compare it with your own listening rather than treating it as unquestionable.