Here's one I did earlier for those lovely people in Texas. They called up (great accents), and asked if I could do a Where's Waldo/Wally type of drawing. There was so much doodling to do in such a short space of time, but it was fun - and how I laughed while drawing. (..but you needed to be there). First I sketched it out, no paper in sight.
Then when they were happy with this, I went over it in 'ink', adding silly details...
Then, when they were happy with that I started colouring it in. A slow.. slow process.
Then finally they made the poster up and plastered Texas with them. I hope it helped somewhere along the line.