I started with my horse backed into a corner with the horses left shoulder near the fence. In the beginning, you may need to lift your horses head to promote lifting his left shoulder so he steps his left leg across and in front of his right leg. By starting with him backed into a corner, he will be limited to how much he can move his back end and learn to move or pivot his front. Start out with one step (the cross-over) and then stop and praise him. If he's not sure what you're asking, tap him on the left shoulder with the end of the lead rope or a crop as you lift his head to reinforce moving the shoulder. Gradually increase the amount of steps you ask for and the speed you ask for the maneuver, but make sure the steps are precise before asking for more steps.