The prerequisite to starting is having a working instrument (roughly) in tune. Then you can start. If you don't know absolutely anything at all, then it wouldn't be so bad to learn some things in parallel to learning by ear, such as basic chord shapes. But you can get started with trying to transcribe happy birthday by ear. Try to play it and find the notes. After a few nusrsery rhymes, you can move on to something simple that is perhaps more inspiring. But to answer your question, you start by taking the first step.
alt Hacker News
The prerequisite to starting is having a working instrument (roughly) in tune. Then you can start. If you don't know absolutely anything at all, then it wouldn't be so bad to learn some things in parallel to learning by ear, such as basic chord shapes. But you can get started with trying to transcribe happy birthday by ear. Try to play it and find the notes. After a few nusrsery rhymes, you can move on to something simple that is perhaps more inspiring. But to answer your question, you start by taking the first step.