A good finstroke is above all a movement that starts from the hips.
We just see too often swimmers bending their knees! This mistake doesn't allow the fin to be 100% functional. In order to know if you're using your fins correctly, try to do some pratice at the pool by swimming on the surface, on your back, with a swimming board on your legs... if your knees touch the board, well that's not good! The finstroke is a very technical gesture that may require hours of training! Feel free to make a video of yourself and try different finstroke durations to find the right movement!