1) Don't excessively bend your knees
2) The movement starts from your hip and the knee bend is only due to the pressure water puts on your foot when you kick downward
3) Relax your ankles and let them snap at the end of the downward kick like hitting a fly with a fly swatter
4) You can rub your big toes over each other as you kick to help with the right movement
5) Your foot is your fin
6) Keep your legs relaxed. The more tense you are the more difficult it will be.
7) You can pretend like you are kicking a football (soccer ball) with a bit less knee bend.