Play the song and hum the note that feels like home, usually the last note of the chorus or the final chord. Find that note on your instrument. That's probably your key. If the song feels sad at rest, it's a minor key; count from the relative major (A minor uses C major's chords).
For the key you found, list the 1, 4, 5 and 6m. In G that's G, C, D and Em. In D it's D, G, A and Bm. In E, E, A, B and C#m. The Nashville number chart has every key.
The melody wanders across every chord. The bass usually plays the chord's root. Hum the low note at each change and match it to one of your four. If the bass note fits two candidates, pick the one that sounds right on the downbeat.
Tap your foot and count to four. Most songs change chords once or twice a bar. Write one chord per bar, and mark where a chord comes early.
If none of the four fits, try the 2m and 3m, then a flat 7 (F in the key of G), then a major 2 (A in G). Those cover almost everything left.
I built Chartless for the songs where this takes too long: a new release with no chart yet, a live version in a different key, an old record that's slightly sharp. It listens to whatever is playing on your phone, tablet, Mac or in Chrome, and shows the key and the chords live, with Nashville numbers. Nothing is uploaded, and the live chart is free.
Got a song it charts wrong? Tell me which one: [email protected].