Izzie; To get the ball rolling, why not try to forget that you are writing a dissertation, and instead begin as if you were writing to a friend you hadn't seen for a while and they wanted to know what you were involved with at the moment.
You begin with telling her who they are and what first attracted you to these artists, explain to her some of their works and why you think this is important. You might have some reservations about certain aspects of what they have said or done, you might say what you find challenging about this, and so on and so on. Before you know it you'll be writing your dissertation, which can be edited over later. The important thing is to get writing, as your tutor has said. Good luck!
P.S. I taught Fine Art, B.A. Hons. and post-grad for 30+ years.