If you know the melody of the tune, that is if you can hum it or whistle it or sing it you are well on the way to playing by ear. Once you become familiar with the notes and scales on the accordion you should be able to reproduce the tunes you know and play them, at first just with one finger, then with other embellishments. Mark Savoy famously wouldn't teach kids to play the instrument until they knew the tunes themselves. You don't have to know any notes, you just need to have the music in your head!