The first step in writing my strip is brainstorming in a notebook. (Actually, the first step is making a cup of coffee, brainstorming is second) I just write and doodle and write and write whatever comes to mind. It doesn't have to make any sense or be a fully formed idea.
I've scanned a few pages out of my notebooks to show you how the brainstorming session works. Click on the image to see a larger version. There are usually a lot of doodles on these pages but these ones just don't happen to have any.