One of the major challenges for all teachers of students with autism (regardless of the subject matter) is getting past the severe communication deficits that are typical of this diagnosis. In a usual classroom setting, the teacher can stand at the front of the class, give information and directions, and have a reasonable expectation that their students will understand what's been presented. Students with autism will generally learn poorly or not at all under these conditions - the verbal information goes by too quickly and is often drowned in a sea of background noise - even when the words are heard clearly, their meaning may not be understood (because of gaps in language knowledge).
If you want to teach a person with autism effectively, you would do better to "show" more than you "tell". Rather than filling the air with verbal explanations, put information into visual format and then model what you would like the student to do. If you are chatty by nature, damp it down - speak in single words and short phrases backed up by your visual pictures and modeling - think of your words as stones dropping one by one into a pond, with lovely silence in the in-between time as the ripples go out and the ASD student can think about what you said.
Art is a perfect subject for this type of approach - you are teaching visual and hand-eye coordination skills that are only imperfectly described by verbal language in the first place. A book that I would highly recommend is Mona Brookes' "Drawing with Children" (and the follow-up book "Drawing for Older Children & Teens"). In the initial chapters of the "Drawing with Children" book, Mona describes a method for teaching classes of young children to draw what they see - she draws a picture step-by-step, describing what she is drawing, and the children draw each line and shape as she does. I have used a variation of this "follow me" style of art teaching for many ASD students of varying levels. One major difference, compared to the method described in the book, involves the language level - talk less than is suggested, and use words that name and describe the pieces of what you are drawing (rather than the shapes and angles). The other major difference is that you will get better results, and a calmer happier student, if you (or another adult model) sit side-by-side with the ASD student, modeling the drawing or painting or sculpting, letting them control the pace of incoming information (don't start modeling a new step until they have completed the step before).
Another type of resource I would recommend for art teaching are the books that use pictures and words to give step-by-step visual instructions for art projects. Ed Emberley (children's author and illustrator) is terrific at this, and he has published many books that lay out visual stepwise instructions for how to draw almost anything. The first time that Adam learned how to draw something from someone other than me, was sitting side-by-side in his school classroom with an adult volunteer (who was an artist herself), drawing animals in a "follow me" mode, using an Ed Emberley "learn to draw" book as a reference for both of them.