The Modern Art of Taming Wild Horses/Chapter 23
Take him in a light stable, as you did to ride him; take the harness and go through the same process that you did with the saddle, until you get him familiar with it, so that you can put it on him, and rattle it about without his caring for it. As soon as he will bear this, put on the lines, caress him as you draw them over him, and drive him about in the stable till he will bear them over his hips. The lines are a great aggravation to some colts, and often frighten them as much as if you were to raise a whip over them. As soon as he is familiar with the harness and lines, take him out and put him by the side of a gentle horse, and go through the same process that you did with the balking horse. Always use a bridle without blinds when you are breaking a horse to harness.