How and What:
1) Hearing live music. As an extension, being trained to play in an ensemble. Are either required? As @whart has written multiple times...no, but the skills can be applied to listening to a home audio system.
2) Listening to a familiar recording across multiple systems, sometimes with the guidance of the system’s owners to be made aware of elements/aspects in a system’s sound. I hate hearing the same recordings over and over, but the fact is the process is extremely helpful when learning to listen to a system, then later when evaluating changes to a system...or evaluating an unfamiliar system.
3) Related to above - listening to tracks on test LPs or CDs: Stereophile, XLO,etc. Why is this helpful in learning how to listen? Each recording on a test disc is provided because it offers an example (or examples) of a particular quality: human voice, piano, venue cues, image depth/width. Liner notes usually explain what to listen for in each. Very, very helpful in the process of learning how to listen.
4) Someone to guide (or teach) during a listening session, preferably in one’s own system so the sound characteristics are familiar. I had a manufacturer once come to my home to demonstrate a component. Toward the end of the demo, he swapped in some footers he liked to use. We also compared to footers I owned. He offered some observations that were quite helpful, and provided a lesson in listening.
Number 4 is important. In my life, when I didn’t understand something, or when I didn’t perceive something someone else perceived, it was always helpful for that person to explain in detail what they were perceiving, and guide me to it. Guide me toward how to do it. When a breakthrough occurred, it was a very exciting moment.