I start out with sketching on paper with pencil
I do some digital sketching in Photoshop and clean up some drawings. While drawing these cars I looked at some real cars to see if my memory had missed anything major. I also have these kinds of thoughts: "I don't know if the mirrors are necessary because the eyes are on the hood . . but if I start thinking like that then what are the seats for? I'll keep the mirrors. What about an engine? Can I have a tongue instead of an engine? Maybe I should go with for an old VW Beetle with the engine in the back? No. It's a cartoon. No one's going to question it."
Before I start work on the final image, I draw little sketches to figure out a composition (here are only a few of the sketches - these can be in pencil or drawn on the computer). I think about things like what angle we are viewing the scene from and what the story needs to show. If this is for a picture book I also think about where the text will be. I then look at pictures of things that will be in the image. These pictures might be of woods and trees and wolves to get the feeling of what they should look like before I draw cartoony versions. If the real thing is easily looked at, I might just look out the window at a car or a tree.