Trailer loading is not a BIG deal. It is just heeding. When you step forward, the horse steps forward. When you stop, the horse stops. Your horse stays by your side always and that includes walking next to you into a trailer.
The thing you never do is force the horse into the trailer. You don't want to make any part of your training program scary to the horse. You just stay at the shoulder and calmly walk into the trailer. Loading a horse in a trailer is not an accomplishment. You don't need to do anything loud when you load a horse into a trailer. It is just having your horse on your heeding aids so completely that he goes with you anywhere. So if your horse does not want to go into a trailer, you need to go back and sharpen your heeding and bring up the trailer question again later.
Fine tune your step cue by going back to the stage of reinforcing what you ask with an aid, if necessary. Walk forward and reinforce the message that the horse is to step with you by tapping its hindquarters with a whip. Stop by turning your body parallel to the horse's body and blocking its chest with the whip, emphasizing the side that matches the forward front foot. Do a LOT of backing, controlling each stride. Get accuracy. Get only the stride you ask for and no more or less. It is accuracy that makes the horse heed you into the trailer. If you don't get accuracy you will get disobedience.
Remember that if the horse does not enter the trailer, it does not necessarily mean that he is disobeying your step cue. He may not quite understand heeding well enough yet. You may need to go back and work more with basic heeding to confirm your cue.
Don't get bossy. Accuracy comes from doing the same thing over and over, not the same thing louder and louder. Use repetition, not retribution. To get the horse in the trailer, you apply consistent directional pressures in a relaxed, rhythmic way. You don't let those pressures get louder and louder or faster and faster.
Everybody wants to see you shake a ladder and crack a whip and put a horse in a trailer in little under a second. That may impress an ignorant audience but it's a negative learning experience for the horse. With really good training, the horse obeys, follows, and stays in the trailer not from fear but from trust. When you create a fight with a horse about something, the fight will become bigger and bigger every time. So it will get harder and harder to load the horse into the trailer. It's a longer and slower process in the beginning to get the horse to respect your step cue and earn its trust. But once you earn trust, you have it for life. In fact, once you have shown a trusting horse that the trailer is safe the first time you load it, it will usually walk right in the next time.
A trailer is just another piece of equipment in the training process and should be introduced with rhythm and relaxation. You introduce the saddle in stages and another piece of new equipment like a trailer should be approached the same way. If the horse is uncertain but paying attention to you, just keep going back to where everything is calm until the place where everything is calm gets closer and closer to the trailer. Do things in front of the trailer that are calm like picking up the horse's feet or doing some grooming. DO NOT hurry. Every time you feel the excitement level rising or feel yourself losing control, go back to something the horse understands how to do well to establish control and calm again.
Some horses may walk right in the trailer with you the first time. Some you may need to coax and cuddle a bit. You don't need to get the horse in the trailer the first day. The process may take weeks. Remember that if the horse does not get in the trailer on the first day, you have NOT taught him that he doesn't have to get in. Horse's don't think like that. They will remember the last thing that happened. So, on the second day, he will remember the spot behind the trailer where he still felt comfortable and you can take up where you left off.
When you get the horse in the trailer, you must remember that since it is a new piece of equipment, he will be a little uncertain. So don't add anything new at this point. For example, don't drive him around yet. Let him stay in and investigate and if he wants to get out, let him. And let him get out in any way he feels comfortable. Most horses will want to turn around and walk out their first time. If your trailer does not allow for this and your horse is accustomed to driving lines, you can put lines on the horse and gently ask him to back out. You may not have a problem at all with the horse backing out. Just don't pressure your horse to back out of the trailer. Don't add more pressure to the issue until everything else is working.
If the horse is afraid and tries to escape from the trailer, do NOT punish him. You must regain relaxation and make the trailer a comfortable thing. So you may need days and days and days of just letting the horse investigate the trailer while you stay at his shoulder and show him that it is safe. Just keep walking the horse up to the last point behind the trailer where it is still comfortable and stay there. Scratch and groom until you can step him forward again and still remain comfortable.
Avoidance situations like whipping or shanking the horse to enforce obedience create up to five times as much activity as approach situations. So an approach situation like coaxing a horse with grain or hay is usually a waste of time when it comes to a situation the horse is unsure of, like a trailer. That's because the drive to avoid the trailer will be stronger that the drive to approach the bribe. However, approach situations (offering grain or hay inside the trailer) will sometimes be what will put the horse in a calmer state of mind. So you don't want to rule out approach situations entirely.
Your first objectives in training are always rhythm and relaxation. And the first indication that you've interrupted the training rhythm is that the horse's breathing pattern gets interrupted. He catches his breath. As soon as that happens, you've got to go back and reestablish the rhythm and relaxation before you can move on.
If your horse is scared, be patient. If your horse is disobedient and wants to fight, change the subject. If your horse is calm and obedient, he will heed your step cue, stay at your shoulder and walk right in.