When I first started working in technical diving, the back kick was mentioned but not emphasized. My buddies and I learned it on our own by watching videos, sharing experiences, and practicing. We eventually realized it is an extremely important kick because it enables you to maintain or get to positions in the water relative to teammates, ascent lines, etc. If I were to do my training over again, I would make this a focus of my earliest training.
The most important tip I can give you is that every part of the back kick is unnatural. It goes against what the body is used to doing, and it goes against what your fins are designed to do. I think that knowledge is useful
Because it is contrary to the design of your legs and fins, every part of the stroke that is in a more "normal" direction needs to be minimized, chiefly through slowing down the speed. (I am talking about the recovery stroke here.)
During the recovery stroke, think about the position of the fin blades. If they are looking like normal finning in their angles, then they will push you forward. Try to get them to cut through the water s-l-o-w-l-y like a knife.
Most importantly, understand that your muscles have never really done this sort of thing. They will at first be uncoordinated, and they will at times seem to be beyond your control. That will change with time. Just keep at it and eventually they will figure it out. Before too long, you will be doing it without having to think about it.