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Ideally you should train your horse to lead when he is still young (I start mine right after they are weaned) because at that age they are far less heavy, making it significantly easier to guide them while ensuring there is far less chance of them accidentally hurting you.
I do not recommend you use a nylon lead when you train your horse; soft rope leads will serve you far better. When your horse bolts ahead of you unexpectedly rope burn is always a possibility as you cling to the lead, and whereas rope burn is bad enough let me tell you that the fairly punishing edges of a nylon lead can really do a number to your hands. Nylon leads also tend to be more slippery than rope leads, and a firm grip is definitely a plus when training a horse. All in all, nylon leads are a terrible choice if you're about to start your horse.
Proper positioning is an integral part of leading your horse in a safe and disciplined manner. You should stand to the left of the horse even with his front shoulder. Do not let the horse get ahead of you, and definitely do not let the horse follow along behind you. Your horse could potentially spook, and if you stand in front of him he just might charge through you.
Make sure there is at least one foot of distance between your right hand (which is holding the lead) and the horse's shoulder. Never let the excess lead length drag along the floor; instead make sure it is neatly coiled or folded. Do not wrap the lead around your hand or arm because if the horse spooks or bolts your limb will be caught and you'll be dragged, making a broken bone very likely. If you want to give yourself added leverage to cling to the lead should your horse bolt, tie a knot at the end of your lead. You want to be able to keep a firm grip on the lead, but allow for an easy release if you absolutely cannot maintain control.
Some horse owners leave no slack in the lead line when leading their horse, almost as if they are afraid to trust their horses. This is a sign that either the owner is misguided or the horse has not been trained to lead properly since a well-trained horse should behave perfectly fine with a loose line. When you keep a death grip on the lead line and minimize any slack you are inevitably expressing distrust or nervousness to the horse, which will be counterproductive to your leadership role. You want to show your horse that you are cool, calm and collected by giving him the line and tightening up on it only when he breaks the rules. The more trust you place in your horse, the more likely he will be to respect that trust.
When leading through a narrow opening you always want to cross through first. Before crossing through make sure your horse is standing patiently, because many horses are tempted to pass through a portal as quickly as possible and you don't want to be run over in the process. If you do not have absolute control near the opening, do not cross through until you regain it.
As mentioned in passing above, always make sure your horse respects your personal space. When a horse becomes nervous (particularly young colts and fillies) his first instinct will generally be to keep close to you since you are his source of protection. While this is an endearing sentiment, as the horse grows it can become dangerous – no one wants a 1000+ horse slamming into our side or stepping on our foot because he was too close!
Make sure you insist that at least a foot of distance be maintained between you and your horse's shoulder. If he invades your space, whether out of stubbornness or nerves, push him off or jab him gently in the shoulder with your elbow.
If your horse's nervousness turns into outright fear then stop him and make him face you. Once he is stopped and facing you, you can approach him and help reassure him that everything is fine. We don't necessarily want to ignore a horse's fear; we just want to help him conquer his fear in a manner that permits you to call the shots each step of the way. By stopping him and turning him to face you so that you can approach him rather than the opposite, the same end result is achieved and you maintained full control all the while.
The remainder of the essential rules to training a horse to lead safely is included in Part Two of our article.