I play several instruments but have never had much training in music theory. I'm an amateur songwriter also. My question is about chord progressions. In section 3.5 (Melodic themes), it talks about when it's okay to "break the rules" in adding melody with chord progressions (i.e. the modified repetition section of the chapter which says in part: "some songwriters will want to use repetition in their melody...a repeating pattern might sound great over one chord but not work at all when repeated over the next chord in the progression...make changes to fit the underlying harmony)
So for example, say you are three lines into the song you are writing... when you have a small section of your melody that doesn't fit with your chord progression, why can't you just change the chord for that measure?
[i.e. When I write a song, I write lyrics first because that's what I'm strongest in, then I write the melody line, then I figure out what chords work with that melody line. So my chords don't necessarily follow a set progression pattern, but I am aware of the basic rules of cadence chords, etc.]
You don't have to start with chords first. However, some writers start off with them (i.e. fiddling with chords until they find something nice), then write melody and lyrics later. It's based on the writer's preference.