the way i approached it was to leave the demo for after i'd already done my first attempt at the assignment and had submitted it. *then* i watched the demo. after that i closed the demo, pulled up my reference photo and made a second attempt and submitted it. i used the demo only as like, a midpoint refresher between the attempts. you shouldn't be following it line by line, or even section by section, that won't help you learn to see the simplified shapes of your reference, nor how to transfer what you're seeing to your paper.

When I went through the course I looked at a demo kind of hand-in-hand with critiques, something to look at after you make your first attempt. IMO you don't need to try to match demos exactly but they are there to help you see an approach that works in case you are stuck or missed something. Doing an assignment again after you see demos or critiques is really up to you as you assess how satisfied you are with your attempt. If you mostly got it there's no need, but there were definitely a couple of assignments where the demo showed me that I had gotten it fundamentally wrong and decided to do it over. Even if you just want to make minor improvements, more drawing & practice is always good!