It wasnt "several years", but it was a few years of self study and then a few months with some other preacher dude.
"After a few years of self-study, and then three months training by an accomplished artist/instructor (a missionary on a one year sabbatical), the skills to accomplish the task began to unfold. These skills were honed by the drawing of wild animals and Christmas cards featuring our grandchildren."
http://www.jesus-withyoualways.com/story.htm