play by ear
play by ear {v. phr.}
1. To play a musical instrument by remembering the tune, not by reading music.
Mary does not know how to read music. She plays the piano by ear.
Joe doesn't need any music sheets when he plays his guitar; he knows many songs well and can play them by ear.
2. {informal}
To decide what to do as you go along; to fit the situation. — Used with "it".
John decided to play it by ear when he went for his interview.
It was her first job and she didn't know what to expect, so we had to play it by ear.
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Source: A Dictionary of American Idioms
