"playing by ear" means to play a piece of music without knowing the exact notation. You listen to it and repeat it from what you just heard. Adapted to your example it would mean to use a computer without knowing how to do it. You had to learn by yourself.

I agree with Mark Kramer. Play it by ear (idiom): it is an informal expression meaning to improvise and decide how to deal with a situation as it develops rather than by having a plan to follow: 'I don’t know what they’ll want when they arrive—we’ll have to play it by ear.' 'When we get into the meeting we’ll have to play everything by ear.' 'He never prepared his presentations. He always played things by ear.'

Domasla, the etymology is correct, but it's rarely used in such a literal context. Nor does it really mean "without thought." It's foremost an idiom, and means "To to deal with a situation as it develops, without a plan; to improvise."

"Play it by ear" is used exactly as domasla said. It literally refers to one's musical talents (to play an instrument without reading music). It is also often used in any situation where one is completing a task without the guide of directions, without thought, or without preparation. Examples: *"Are you going to read the directions first?"...."No, I'm going to 'play it by ear'" *"I don't know if I can meet with you for coffee this weekend. I am very busy. Let's just 'play it by ear' and see what happens."