People often ask how a song is written, as if there were a door one could simply open. In my experience, a song is not written. It is uncovered, layer by layer, with care and patience.
The first line usually arrives uninvited, often while I am doing something else: stacking firewood, walking the dog, listening to the rain. It is rarely impressive. It is usually just a phrase that refuses to leave.
Then comes the long work of accompaniment. The melody must be found, not chosen. The chords must support the words without explaining them. Each section asks a question, and the next section must answer it, or the song collapses under its own weight.
By the end, if I am lucky, the song knows more than I do. That is the moment I stop rewriting and let it go.