For me, something will come in my head and I'll either end up calling my cell phone to record it, or I'll just pick my guitar up and see what comes out. Sometimes it sucks, sometimes it doesn't. So there's really no set method behind it.
The thing I'd really like to see is the old London Bridge, with all the old buildings around it like Shakespeare's Globe. I'd like to walk along that. Don't worry, I won't get drunk and fall in.