All great tips.
One thing I know that all great songwriters learn are these Three things:
1.) Getting past fear and self doubt
2.) The power of mumbling and the power of crazy
3.) Singing and playing for joy
All three are critical I think.
You should learn as much about theory as you can stomach. Paul Simon says that without a strong foundation in chord theory you won't be able to get yourself out of those little corners you inevitably paint yourself into during the course of writing a song.
But when you sit down to write, that should go completely out the window, I think. You know this stuff, but you don't write to a formula. You write from your gut and just use theory when you run into a traffic jam and there's no way out.
For example, I think just sitting in your room strumming a guitar and mumbling until something pops up is a great way to go.
I once listened to an outtake of the Stones working on "Miss You." They recorded the entire writing session which was like 8 hours long and Mick was babbling incoherent things the entire time over top of Keith's riff, stuff like "Walking like a lark, lighting up the park" and so forth.
Then finally he went "And I miss you..."
And you could hear Keef in the background yelling. "That's it. That the title."
There is an awful to be learned from that I think.
Oh and Jimmy Page. Here's a guy to study.
Not many people know this, but Jimmy Page is a chart reading monster. He was the #1 session guitar player of his time in Britain in the early 60s. He recorded with Burt Bacharach on the instrumental versions of some of his most famous stuff and recorded with countless other famous icons in his early years. He was legendary for never having to rehearse. He would just look at a score and go "I got got. Let's cut this thing."
BUT--I highly doubt he worried about any of that when he played the guitar solo in Stairway to Heaven. That came from a whole different place man.