The Typical Psyche Fallacy says my methods won't necessarily work for everyone, but anyway...

The hardest part for me was the beginning, getting a toehold at any inner sound. Pick a note on the guitar - I started with D on the second string. Play it at a steady rhythm with rests, slowly fading away into nothing. (Might not be possible on the piano or other instruments.) At some moment the brain will start to "complete" the sound, even though by that point you're playing too softly to hear. Catch that feeling, expand on it. When you can "do" several different notes, try playing a simple melody and hearing it afterwards. After you're comfortable with that, try to hear a simple major scale without playing it immediately beforehand. Then work from unfamiliar sheet music without playing it - solfege-sing in your mind - by now I can do this quite easily. And so on.