First things first, pick a school bag that works for your child and the level they are at in school. Books are heavy and a well fitted school bag is important for posture and comfort.
Once you have the school bag sorted, decide on a lunch box and drinks bottle, bearing in mind what pocket space you have available.
I was always a last minute panic school bag packer! My Mum would nag me for weeks before I went back to school to get organised and now I find myself doing the same with my own children. I always seemed to manage to have my school books (thanks to my Mum), but beyond that it was a bit hit and miss!
So my advice would be that a few weeks in advance of returning to school, check you have all the school books on the booklist and get them packed into the school bag. Then you are ready to get onto the other essentials that are needed.