PROCEDURE
There are multiple methods to teach a horse to trailer load. Generally, no devices are needed other than a halter and lead rope, a trailer that is of adequate height, space, and in good repair. It is also helpful to have an assistant (with the same understanding and goals) help out.
Park the trailer appropriately. The trailer floor should be functional and sturdy. If a ramp is used, it should be functional and safe. Orient the trailer so that the interior is bright, and ensure the trailer door is secured open.
The key to trailer loading is pressure and release. Before you ask the horse to load, ask the horse to yield to pressure on the poll with the halter and ask them to step forward from the side (lateral lunge). If they do not do this, then they may not trailer load either. Asking a horse to trailer load that does not understand this pressure and release is like asking someone who cannot read to repeat a paragraph. if they have learned the paragraph by memory, they may do it. But if they forget the words, there will be no way for them to figure it out again.
Stand on the driver's side of the open trailer door. Reach your left hand toward the interior of the trailer and put pressure on the halter. Raise your right hand. The horse should step into the trailer and continue walking to the front. If this does not work, then you may consider working from the inside of the trailer. To do this, step into the trailer and ask with some pressure. If the horse pulls back, apply more pressure. If the horse moves even a bit toward the trailer, immediately slack off and allow the horse 10 seconds to process what has happened. Then ask again and repeat.
There are many resources for trailer loading and many different techniques. Regardless of your technique, follow the 2 minute 30 second rule. There is rarely a reason to spend hours trying to load a horse.
TIPS FOR SAFETY AND SUCCESS
This is not an easy skill to describe. Your ability to load a horse is related to your ability to get a horse to do many other things. If you lack confidence, skill or good timing, trailer loading may be difficult.
Practice this skill when you do not need to transport your horse anywhere, and when you have the time and patience to do it right. The time to teach a horse to trailer load is not when you are faced with an emergency.