I decided to document the process I use for illustrating.
#1. I made a sketch and cleaned up the lines.
#2. I put down the colors quickly, like an underpainting.
#3. I went back with markers, pushed back and pulled forward different parts to create depth, and layers of color.
#4. I added some colored pencil for texture, and an outline to preserve the "cartoon" look of the boy and the dog. I also added the title on Photoshop.