Doing a back wave is one of the floor work challenging moves in belly dancing not simply because the technique is difficult in itself but also because many of us struggle with knee and back issues these days.
So, follow the steps here to see how to get started and I will be posting new videos later to continue from where we left here. This is a move you really want to learn slowly. One step at a time ... to void injuries. So please do NOT try to go fast if this is your first time doing it. I am speaking from experience here :)
Good luck and be safe!