At least the way I learned it, there is a LOT more motion at the hip and knee in a breast stroke kick, than there is in the diving frog kick.
The mechanics are pretty simple: Starting from the horizontal position, thighs straight out behind you, knees bent about 45 degrees, fins parallel to the bottom. Separate your feet; most of this is rotation, with a slight increase in distance between the knees as well. Keep fins parallel to the bottom while you do this.
Now rotate your ankles so the bottoms of your fins are facing one another, and try to clap the bottoms of your feet together like a seal does its flippers.
The big traps are: Bending at the hip, too much separation of the knees, and trying to bring the TOPS of the feet together (which is what I was doing). Not keeping the fins horizontal during the loading phase is problematic as well, because it provides a ton of drag.
The biggest problem with trying to learn this stuff by yourself is that you cement bad habits, and they can be hard to break. I am very glad that NW Grateful Diver, who was my early mentor, told me simply to use a small modified flutter kick from the ankles (relatively difficult to do wrong, so long as you keep your body flat and your knees together) instead of trying to learn the frog kick on my own. Steve White, my Fundies instructor, had me on the right track in one day.
