Depends how deep down the rabbit hole you want to go. Nothing beats hours underwater, as has been said. But I think there are certain things that can be done to speed up the learning process a bit, for someone really eager to get better.
Here's a few things I did when I just started out:
- I train swimming weekly, for exercise and to improve comfort in the water and water feel
- Every time I was in the pool, I would practice back kicks with no gear on the surface and underwater for 5-15 minutes
- I have trained Pilates, which is very good for alignment, posture and body awareness
- Sometimes I would lay flat on the bed, on my stomach, with the feet sticking out, practicing/visualising kicks
- I have even practiced the back kick while sitting on a chair: focusing on collecting feet, pointing/stretching ankles together, extending legs fully, flexing ankles to initiate back kick motion
Most of this is probably completely overkill and not necessary, but I think they each helped me gain better awareness over rotation, ankle position and similar details that have probably improved my finning techniques.
