A couple of things:
In general, with the superman plank, you probably want to position your legs (and arms) a bit wider apart, to make it easier to stay balanced. The best way to figure out what’s right is to just try it for yourself and experiment.
There are different versions of the Superman Plank.
“Static On-Hands Superman Plank” (I made that up myself)
You get in the standard plank pushup position.
Then you raise one leg up behind you, so that it’s completely horizontal, and raise the opposite arm in front of you, so that it too is completely straight and horizontal. Keep the position for 1 minute. Then take a quick break and repeat on the other side.
By doing this on your hands (instead of on your forearms), you’re engaging your core anti-rotational strength more.
“Static On-Forearms Superman Plank” (I made that up myself)
This is the exact same exercise as the Static On-Hands Superman Plank, except that you do it on your forearms (instead of your hands).
Dynamic On-Forearms Superman Plank
You get in the standard plank pushup position.
Then you raise one leg up behind you, so that it’s completely horizontal, and raise the opposite arm in front of you, so that it too is completely straight and horizontal. Hold the position for 2 seconds, and then get switch sides, hold for 2 seconds, and repeat that until you can’t anymore.