How do you train your ears?

The only way to establish an unbiased frame of reference is to go to listen carefully to live performances of various types in various venues.

For getting started (complete beginner) I'd suggest the basic audio course by Alton Everest: "Critical Listening Skills for Audio Professionals".

I agree with Sidssp. Probably the worse thing you can do is listen to most recorded music. That is the idea of what the recording engineer had in mind. Put that bias into a stereo system, many of which has different brands of players with their manufacturer's biases, amps,with theirs, speakers,with theirs, cables,with theirs, etc., and if some idea of real music comes to you, it is just an accident. The real thing unamplified should be the standard.

"The real thing unamplified should be the standard. "

Of course the real thing should be the standard. Everything else is just a reproduction of the real thing.

Some genres are amplified naturally though in live performance. That should be considered a real standard for those genres in those cases as well.

No recording or playback system, regardless of cost or quality, should be regarded as a standard for purposes of training the ear. For purposes of providing a model for a good playback system, yes, but that is different.

I agree with listening as much live music with real instruments would help. I had purchased several CDs after attending to live jazz concerts, but always it was disappointment to listen to their music on my audio system. There is just no comparison between well played live music and recorded music, unless done extremely well.
Orchestral music can be hardly reproduced in low-mid high end systems, however well recorded.

In a sense, training ears would be similar to training wine tasting -- you need to listen to as much real live music as you would drink lots of wines to know more about wines; reading books on wines would help to a certain point, but no theory would substitute the real tasting. However, tasting wine with suggestion made by some wine gurus, like Parker, would certainly help your wine tasting.

IMO, audible memory in music is rather weaker than visual or taste memory. But it can be further cultivated as with wine tasting. Everest's book may help. I will check it out.