Sculling sets the foundation for any swimming stroke, for treading water, for any sort of exercises or recreational swim that you might be doing during the time of swimming. Sculling pretty much is a movement of palms and forearms, that create propulsion for you to stay on top of the water and also give you the feeling and the efficiency of the strokes. There are a couple of ways to do sculling.
First, you can do sculling by going on your belly, keeping your face in the water, and having your arms at a 90 degree angle, and having your palms rotate in and out of the water to help you scull and propel through the water. You want to minimize any sort of movement that might occur from your shoulders, or more from your arms, because then this is not becoming sculling, it is becoming more of a stroke exercise. Sculling can also be done with your arms forward, in front of you, to move forwards or backwards with your feet first, or with your head going forward. And, sculling can be done with your arms at your sides to move your feet first, or your head forward.
A great way to practice sculling is to begin with your arms in front of you, and just scull all the way through underneath your body as you're on your belly. As you get down to your hips, recover your arms close to your chest, up, back to the original position, and again, scull all the way through by moving your palms through the water to make it more efficient.