I'm going to deviate slightly from the format, but I just posted this on PSR Tutorial Forum on a thread about "playing by ear", and it seems relevant.
Jogging my memory about how I learned to "play by ear". BTW I hate that term. My kids aren't interested in doing what I do, so maybe I can share some of the process on here and help someone along.
1. Started playing trumpet so learned to read the melody line. Can read any treble clef music. We always had a piano, but all I did was tinker around on it, picking out melodies. Started relating trumpet notes to piano notes.
2. Started playing organ in a band (learn as you go), so learned to play basic chords, C, F, G, E, A, D with left hand. I took Music Theory in college and this certainly turned a light on, learning the relationship between the notes that make up chords, the intervals, etc.
3. Started doing some solo work, so used music sheets or Fake Books, either bought or home-made, with the chords and the melody line. Most were in C.
4. Gradually learned to deviate from the melody notes I was seeing and adding fills and runs. I learned them by listening and imitating organ and guitar players.
5. Got tired of paying a singer, so I started singing the melody notes instead of playing them. Still did the fills.
When learning a song, I played the melody line and sang it at the same time. That's how I learned to sing. This also taught me to learn the songs in appropriate keys to fit my vocal range. I learned there were certain singers who's songs I didn't have to change the key to do (Merle Haggard, George Strait, Jerry Lee, Elvis). I learned there were certain songs I shouldn't do at all.
6. As my repertoire grew, I made a LOT of my own lead sheets. Just lyrics with chords over them. By this time I could play most melodies without needing to see the notes. I would record the songs off the radio (remember those), on reel-to-reel, 8-track, cassette, VCR, minidisc, and finally digitally, as the various technologies evolved. Then start and stop, rewind, pause, etc. while writing down the lyrics and figuring out the chord changes. By the time you finish a song, you pretty much know it.
One of the things I did right was to make a lead sheet for EVERY song I ever learned. Started out either writing the words and chords by hand, or on a typewriter. Over the years my pile of fake books grew to something like a dozen packed loose-leaf binders.
7. Along came computers and I started doing them on computer, using word processor. I also started going back and laboriously entering every song I had accumulated into the computer, all by typing the chords and lyrics again.
This process took a couple of years to actually complete. PDFs were not invented yet, or at least I didn't know about them. I also made notes on each sheet as to A. the basic style, (4/4, 2/4, 3/4, 6/8, shuffle, rock). . . B. the bass line (2/4, walking, etc.) . . . C. the tempo (beats per minute). And D. the date the song was recorded and the date I learned it.
When I enter them into the computer I use the Properties to identify Artist, Genre, Tempo, Date, etc., so they can be easily sorted when looking for a particular song, as in doing a request. I might get a request for a George Jones song. No problem, just sort by Artist and the list of Possum songs is displayed alphabetically. "Play a Waltz" . . .just sort by Genre and the list appears sorted by Waltz, Rock, Blues, Country, Standards, Cajun, Jazz, Latin. . .
Now, my repertoire has grown to a couple of thousand songs, at least, and still learning new ones each week. Still listening to good players and continuing to improve my instrument emulations. The vast majority of songs I do regularly I don't need to look at anything except the audience. Actually if I think too much about what words are coming up or what my fingers are doing, I'm more prone to mess something up.
The learning process is much easier now. Find the song on Youtube, play it back a time or two, then play along with it on the keyboard, going through the process above. You can also search the internet for lead sheets some one else has done. Be careful, they aren't always right, but they are getting better. Change as needed, adding Properties and side notes.
So, YES, I can sit down and play just about any popular song, especially if I listen to it a few times and it helps if I like it. Friends and audiences are amazed because it looks so easy. It is now, but trust me, it was a LONG learning process. I've been doing this for more than 50 years and still learning!
Hope this helps someone in some way. It has kept me from having to get a "real job" for most of my life!
I no longer ever look at notes, even when learning a song, although I have an extensive library of PDF files, and a closet full of fading fake books.