there have been several sugestions to dial the stitch length to "0" this is not necessary as when the feed dogs are dropped the stitch length is nutralized. Stitch length is controled by speed of the machine and the movement of your hands. I suggest you find a book on free motion or machine quilting. They will usually have exercises. But generally in beginning free motion. it is good to practice, loops, meandering, a design like waves that has curves ending in a point and then long wavy lines.
The important thing is to work until you feel a good balance between the speed of the machine and the movement of your hands. and don't ever let your hands leave the quilt while the machine is going. This causes jumps when you stop and begin.