I write almost everything I have written, while I'm driving. That is usually when my mind is the calmest.
My wife has an old check register that she says she can never get rid of. A lot of time, when a song was complete in my head, I would dictate the lyrics to her and she would write them down while I drove. I write in my head, and I write lyrics and melody at the same time. The melody is what allows me to sing the song over and over in my head, until I feel it is what I want. By the time I pick up my guitar, I will have sung it to myself thirty or forty times. I really don't know why this method works for me, but thankfully, it does. I have never worked with a cowriter for that reason. How do you tell someone else what you hear in your head.