Talk quietly until you can move the lines up and down the legs without the horse moving around. Drive in this manner around the pen or up and down the barn hallway. Don't use a steady pull, but pull and release on the lines. The helper may have to stop and back the first or second time. You will be surprised to see how quickly a young horse will learn to do this on command.
Line drive the horse for one or two weeks.
After the first time or two, you will not need a helper because you will be in complete control. Drive the young horse inside and outside the pen. Driving will help the young horse relax, and you will be getting a good head set without hurting the horse's mouth. Begin riding When the horse is driving well, you are ready to start riding. Unhobble the stirrups and put the running martingale on again.