There was a significant learning curve, and I am still learning. I suggest, to begin with, enroll in an art class with the person you are supporting, geared to people with intellectual disabilities and observe how the instructor teaches. When I started sitting in on art classes, I learned how to break down an art project into small steps that gradually led to a finished project.
By working very closely with Rita, I learned that some of her challenges are language-related; e.g., she misunderstands the instructions given — so my role is to ensure that she understands the meaning of any instructions.
Some are perceptual issues. Rita may not fully understand shapes, perspectives, sizes. I must develop ways to address these topics with her within a project. Websites designed to teach drawing to children can be beneficial in deciding on an approach.
Fine motor skills are also a challenge. Again, workarounds are needed; e.g., if Rita draws a shape, I can cut it out for her, or we can use templates of various kinds to create shapes on a page.
Also, I had to educate myself on the art materials and the technicalities of using them. Reading articles, watching instructional videos, and talking with the art teachers are ways to become more knowledgeable about materials. Once that information is in place, I can better obtain the right materials and teach Rita to use the materials properly.
This process takes time, but it can be done bit by bit. Each person is different, but by working closely with the person, one can begin to address their support needs.