how do you cartoon a regular picture?

just curious how you take a regular picture and make it look like it was drawn with markers and make it look like a cartoon. I know how to make pictures look like a sketch and how to make em look like they were color penciled, but i was just curious how to make it look real clean like it was drawn with markers because it has the sharp edges and subtle color changes. This picture belongs to nick (crochface) and i am just posting it to give people and idea of what i am talking about. I am just curious as to what is involved in making a picture turn out this way?