I started designing with a few keywords and loose forms in the upper right corner. After that I concentrated on the forms of the heads - because in my opinion faces are the most important part. After that I sketched the whole characters in a size comparison. 

I turned the pencil sketch into the lineart with a few alterations and printed it for coloring.

After coloring with watercolors and colored pencils, I scanned it back in and did some color corrections.