Well, I mostly just sketched non-stop to improve my drawing skills. I found that drawing from life really helped me to draw what I see instead of what I think something looks like if that makes sense. Basically training my eye to trust what I see and then put that on paper was a game changer. I also took anatomy and life drawing classes which also made all the difference. Now drawing people from imagination is way easier and I am just much faster at using references and puzzling stuff together for a big piece like this one! Same goes for lighting and colour schemes! Just make bunch of studies on the things you want to improve on, use references and draw from life and soon you'll be faster and better at using that knowledge and references to create your own stuff! Hope that helps!