Good luck on your assignment buddy, I practice mindfulness when I can. But the whole sitting still part has got to be the most challenging for anybody with ADHD. I was diagnosed in my tween years and haven't taken meds for it since then, I only can pay attention to stuff I enjoy doing. Just try your hardest to force yourself to sit still... You don't have to only think one thought, I usually think many thoughts while I'm doing it, what I do is I talk to myself in my head by describing things I see happening around me, even if its something just as simple as "the carpet is red" "there's a voice in my head" or "I'm sitting in a chair, there is a bug over there" these allow me to stay focused on the present, it helps reduce my anxiety and makes the racing thoughts slow down a bit. Start off small, do it for as long as you can and work your way up to 20 minutes.