And what we're learning is the process of songwriting. Songwriting as in composing music, not composing words. Although they do kinda connect eventually, unless you're all-instrumental.
In this case, we start with a melody, which makes us very old school.
If you don't read music, you still know that song. I guarantee it. (Or your money back!) It's "Twinkle Twinkle Little Star". And the first step is to figure out what key to put it in. That # symbol means there's one sharp, which puts it at G major. Well, it could be E minor, but we're starting with a G, so G major is a good assumption.
So, we have a melody and we have a key. Isn't that good enough? No, not quite. We need to create harmony. Simply put, chords. So, let's put some chords in for a first pass. First two measures only.
G G D D E E D | C C B B A A G
Try it. Doesn't it sound bad? There are reasons for that. We'll try a few more ways.
First, go back to the key. G. The G scale is G A B C D E F#. So, let's pull a few chords. G is G B D. C is C E G. D is D A F#. Between those, we have all the melody notes in the chord. Going back to the first measure, it was the E major that sounded bad. The E note is in the C chord, so we'll use the C chord. Bs go to G, and As go to D.
G G D D C C D | C C G G D D C
D D C C G G D | D D C C G G D
G G D D C C D | C C G G D D G
That's a way to do it. This is why there's 3-chord rock. As long as you stay within the key, three chords is all you need.
But not necessarily all you want. Woody Guthrie says if you use more than three chords, you're just showing off, but why are you playing in front of people if you aren't gonna show off? That second measure goes C C B B A A G, right? Drops like that are everywhere, and I'd likely play that like this:
C C G/B G/B Amin7 Amin7 G
You get the C. G/B is just the G chord with the B, the 2nd fret of the A string, as the root note. We could write Amin7 as C/A (except that's a bit weird, as A isn't part of C), then G.
You can do similar things with the rest of the chords, too.
I said this is old school, and now I should explain. You listen to traditional Irish music, for example, and you hear lots and lots of "tunes", melodies to work from. The backing musicians choose how to play along with the melody, and it isn't written in stone. Listen to John Doyle and you'll hear improvisation and variation in the harmony playing that's just great. And, if you're anything like me, you'll lose track over which part of the melody is "The Kitten and the Cow", which part is "Fredrick's Jig", which part is "The Greater Storm" and how they all connect. But you'll like it all anyway.
We get into more modern music, we find that musicians don't like memorizing dozens of chord structures. As we've established, the harmony is established in relation to the melody, so when you improvise over (older) jazz, you have to choose a chord note. (Consider the big jazz chords. Then consider Freddie Green's chords, which are three notes at best. They write A13 to give the soloist more choices, and choices in the second octave that aren't in the first, not as much so you play a chord with A C# E G B D and F. There aren't enough strings to play all that anyway.) Take one chord structure, like the 12 Bar Blues or the Rhythm Changes or Pachelbel's Canon, throw different melodies on top, and you've got yourself a new song, but something that's recognizable and easier for musicians to remember. "Twinkle Twinkle Little Star" is also the Alphabet, remember. Bob Brozman has a page where he talks about the evolution of the blues form from the early and rough stuff to the very sophisticated and jazz variations, which is very interesting.
There are big books and websites of stock changes. Song recycling is still a thing: "Knocking On Heaven's Door" became "Helpless" became "Fade Into You" without even a tempo change, but I'd be hard pressed to name anything that goes I bIII IV bVII besides "Smells Like Teen Spirit".
I'll do Remedial Songwriting II once I teach myself more. I don't have a clear handle on chord resolution yet.