Sat in sessions, listened and tried to follow.

Prior to that, I could read music and given some time could learn to play things from memory. Even now my ability to retain what I've learnt by ear is limited - in a session I may (sometimes) be able to play along with a tune someone else is doing after three times through, but 5 minutes later I've forgotten every note of it. Repeat every week for a month or two and it'll start sinking in.