Hook the trailer to your truck and drive to a level spot where you have plenty of area to work, such as inside an arena or pasture. Close the front and side doors, and open the roof vents to allow for air movement. Remove any feed because that will be a distraction from your cues.
Open the back doors, and stabilize the divider. If you have a slant-load trailer, clip the partition open so it doesn't move around. With the trailer ready and preliminary training done, you're ready to approach the trailer.
Begin away from the trailer and ask your horse to go forward toward the trailer. Pet him the moment that he steps forward.
As you approach the trailer, there will be a spot where he stops, perhaps 20 or 30 feet from the trailer. Allow him to stop, and pet him there. That's the outside edge of his comfort zone right now. He's done everything right, and by allowing him to stand, you're telling him that he's safe there. If things get too stressful up at the trailer, this is the place you'll come to practice your cues.
When you feel that you have a 90% chance of success, ask him to walk forward. He'll likely go a few steps and stop again. That's okay. His comfort zone has stretched, and that's a good thing. Again let him stand there and relax.
When you feel that you have a 90% chance of having him walk forward, ask him to step forward again. Use that process all the way up to the trailer.
Along the way, the horse may show the beginning signs of one of the evasions discussed earlier. Don't get mad at him. Use one of the solutions to show him that isn't the option you want, and continue with the lesson. Just take each movement, one at a time. If he makes a small move, for instance, to pull away to the right, realize that he's likely to make a larger move of the same type at the trailer.
Ready to Load
Walk the horse up to the trailer or the ramp, and point his nose into the trailer. Your left hand will keep the nose pointed into the trailer, but don't use it to try to pull the horse into the trailer. The horse's back end will tell his front end to step forward.
Allow the horse to stand with his nose facing into the trailer and pet him, just as you did when he was 30 feet from the trailer. You want the horse's head at a relaxed level, somewhere about the height of his withers. If it's higher than that, chances are that he's thinking backward, rather than about going forward. Ask him to drop his head and pet him.
When you feel there's a 90% chance the horse will step forward when you ask him, use the go forward cue. All you want is one step, and chances are that it will be a forward step with one hind foot.
After one step (or two steps if he volunteers the second one), ask the horse to stop. Pet him and allow him to stand.
After a few moments, pick up the lead rope and pull it lightly toward the horse's chest to ask him to step back. Release the rope the moment he leans back and pet him when he makes the step.
When you think he's ready, ask him to step forward. Again, you want just one step. Do not allow the horse to walk into the trailer.
The horse may not take a step toward the trailer. He may drop his head or perhaps paw the trailer. That's fine. Give him time to check the trailer out. Some horses paw the ramp or floorboard several times before they even think of stepping on it. Those are good, forward thoughts, and you'll want to reward them.