Horses are by nature curious animals, and are likely to wander off if not securely tied. As a matter of safety, it is imperative to be able to tie a horse with a quick-release knot, allowing the animal to be untied with one hand should he panic and try to pull back to free himself. Here is how to securely tie up a horse.
First of all you will need to find a sturdy post. The post (or rail) should be secure enough that if the horse were to pull back, it would not come loose. Should it come loose, you risk injury to the horse and possibly yourself.
After finding the right spot, loop the end of the lead rope over the rail. You'll need about a foot of rope to safely tie the knot, but make sure you do not have too much length between the horse and the rail, as he could put a foot through the loop if he puts his nose to the ground.
Take the excess end in one hand and double it over on itself. Cross that over the end that goes to the horse as if you are tying a simple knot in you shoelaces. But instead of the whole end going through and making that simple knot, you will have the looped end as part of the knot with the excess hanging free.
Pulling on the end closest to the horse should not make the knot come free, but should tighten it. If you pull on the other end, the knot will fall free.