It's pretty hard to learn the back kick from a video.
Key is to move slowly, there's no rush. Make sure you're trim side-to-side. If you're rolling to one side or the other while trying to back kick, the back kick will accentuate the roll. May sure your head is up during the entire kick. In double tanks your head should be resting back against the manifold. If your head is down when you kick, your feet will go up and your head down.
First the setup. Bring your feet up to your butt with the soles of your feet touching each other (or as close to that position as your anatomy can achieve) in a kind of frog-leg position. Slowly extended your feet/legs backwards, keeping your soles together and toes pointed backwards. This position gives you a pointed wedge to push backwards. This also has got to be done slowly and smoothly or it will just push your forwards. I find it helpful to get your legs fully extended before trying the next step. As you get better, it will become easier and easier to setup without pushing yourself forwards in the process. Move as slowly as it takes to extend your legs without pushing yourself forward.
Next, keeping your head up, roll your ankles outwards and sweep outwards and forward. The exact maneuver is hard to describe, but remember at this point you're trying to sweep your your fins forward so that you move backwards. Keep you head up when you sweep.
Once you get the hang of the basic back kick, there are a host of variations. You can do a modified back-kick from the modified flutter position to reduce silting. You can do a modified back-kick from the modified frog position, also to reduce silting. It doesn't have to be a full leg extension maneuver. But it's easier to learn the full leg extension first version first, and then move onto the other variations as you get more adept at the back kick. Some people even use the side of their fin for power instead of sweeping the blade of the fin.
You can try it with your hands just to get down the concept. Put your palms together and bring your hands back to your breast bone. Extend your arms straight out in front of your, palms together. This is the setup. Then sweep the back of your hands outward. Get the idea?
You can practice the back kick in the swimming pool, but I personally found this is a good way to practice but not a good way to learn. To really get the feel of the back kick you need to be sumberged, not on the surface. You also really need to be wearing fins in order to learn how to avoid pushing yourself forward during setup.
