Lowering feed dogs is a must. Adjusting tension won't help your jagged circle. Only practice will. I have never had anyone in a class who could just pick up and do it well. You must get a rythm to the movement of your hands. An even flow. And above all work to find your personal balance between the movement of your hands and the speed of the machine.
Don't attempt to go fast. The faster the machine goes the faster your hands must move. Slower hands.....slower machine. Begin by practicing e's, the go to meandering, that is not crossing a line of stitching and then to an arc with point like an ocean wave and then just a wavy straight line.......all these are used in free motion quilting. and practice, practice.