Most music I learn by listening to the melody enough times that I can hum it. If I can hum it unerringly, trying to match the original as closely as my ears allow, I play it on my bowed psaltery. Often, I can play it through the first time.

Hey! Isn't that the Professor Harold Hill Method?! La de da de da de da~da de da~da de da...

When I'm learning from a book, I stumble through it a lot more~unless I suddenly realize, "I know this song!" Then, I play what I hear in my head. The next time through, I see if my memory matches the book's notes. Sometimes, the version in the book doesn't please me as much as the version I've heard. Then, I go with playing by ear. If I like both versions, I learn both of them.

It's much easier for me to play by ear than to battle with a book, especially when I have to turn a page in the midst of things.

Later, when I'm playing in public, I usually stick with what I learned although I can add some flourishes when necessary. For example, somewhere I picked up a bit of melody to add between playing Coventry Carol, meaning: I play it through, play the "bridge", then play it through again, ending on a slightly different note. I try not to stray from how I learned a piece.