I think my husband learned the back kick the best way possible. Go to a pool in a swimsuit, with a mask and snorkel. Lie on the surface and try back-kicking with the tops of your feet. It won't take long before you can move backwards (I've seen kids get this in less than five minutes). Swim laps backwards until it's absolutely reproducible.
Now do it with fins, still on the surface with mask and snorkel, until you can duplicate it with fins.
Then go diving, and do it in full gear.
One caveat -- back kick is very trim-dependent. If you are head-down, you'll do the backwards shrimp dance (back kick yourself upwards). If you are feet down, you'll kick up silt but not go anywhere. Horizontal, you'll go backwards.
Now do it with fins, still on the surface with mask and snorkel, until you can duplicate it with fins.
Then go diving, and do it in full gear.
One caveat -- back kick is very trim-dependent. If you are head-down, you'll do the backwards shrimp dance (back kick yourself upwards). If you are feet down, you'll kick up silt but not go anywhere. Horizontal, you'll go backwards.