Typically there are little fragments of specific words and images swimming around in my mind, and then at some point, I'll sit down with the guitar and everything will fall into place. It's like your brain is a drain with a bunch of words and images dropping into it, swirling around. The drain is stopped up, but you can feel these things dropping into it. Then at some point, someone comes along and pulls the plug out of the drain and everything comes together in the song.