On a practical level, I tend to combine the methods suggested, as I think many people do: I often start by hearing a song (sung live, or on the radio, CD, etc) and think I might like to learn it. I then look in my books to see if I have any versions there, and/or trawl the Internet - starting here, of course - for any further versions or information. I may need to check out the tune as well, if it's one I don't know and can't remember. If the tune I want is not in a book or on Mudcat, Youtube or Spotify can often help.

Sometimes I just pick up a book, find a song and learn the tune and the words as written.

With traditional songs, I often gather a few versions of the song and fuse them together to produce one that will work for me. With songs that have a known composer, especially a living one, I try to stick to what was written, but it's surprising how quickly the "folk process" can change this, for better or for worse, so even in these cases I may check out several sources and edit a bit.

As Vin2, above, has said, most of us also have songs tucked away that we learned orally in childhood, but in most cases I don't have a whole, singable version of these, and would need to refresh them a bit, using the above methods, if I were to perform them.

Marje