I am assuming your trailer is in relatively good condition.
Begin by hooking it up (Do not start teaching with the trailer not hooked to your truck) It may rock or even worse tilt when weight is added)
Drive your trailer to an enclosed pasture (in case your horse gets loose while you are teaching trailer loading) Open the trailer door 90 degrees. This will create a natural barrier to one side of the trailer. You will be the barrier on the other side. Open any windows in the trailer to allow more light inside.

Check the following:
A) Is your trailer large enough for your horse? It is really not a pleasant task to try and load a warmblood into a quarter horse trailer.
B) Is your trailer wide enough that when your horse does go in he/she can turn around and come back out. We will be backing our horse out later but first let’s build confidence and then competence at going into and out of the trailer. We will allow the horse to turn around (at first) but not allow him/her to run out – if possible.
C) Did your trailer recently haul other animals such as goats, llamas etc.? If so the residual smell may scare your horse.
D) If there is one, can you remove the bar where the door closes . Remove any partitions in the trailer. You want the trailer to look as much like a stock trailer as you can. Remember I said horses are claustrophobic so we want to give them as much space as possible.
Before you even think of asking your horse to go into the trailer begin to ask him/her to lunge about 20-30 feet from the trailer. The distance you begin at will be determined by the actions and body language of your horse.
Horses CAN NOT lie. Frame of Body is Frame of Mind and good trainers can read these signs and make adjustments. And so can you. For instance, if your horse changes gait as he/she passes the trailer or if he/she tucks or swishes his/her tail, or if the ears go back or if he/she bends towards you while passing the trailer door read these signs. Your horse is talking to you. The adrenalin that is being produced initiates the fight or flight instinct and should tell you to  move further away from the trailer when you begin to lunge him/her as there is very little learning going on at this point.
When your horse is away from the trailer add a little pressure with your horseman’s stick and remove all pressure as your horse approaches the trailer and  passes the opening of the trailer. We are trying to make the area in front of the trailer a place
of comfort. The goal is to get your horse to maintain a walk for 360 degrees and NOT speed up between you and the trailer while maintaining the correct body language.
It bears repeating. Look at the frame of your horse. Is he/she relaxed? Is the head level or inverted? Is the tail tucked, loose or swishing? Are the ears attentive but not laid back? Can your horse maintain the inside bend on the circle with the head towards you and not pulling on the lead rope when passing the trailer? AND does your horse stay the same distance from you as he/she gets closer to the trailer?

When your horse finally walks past the trailer THEN you can move the lunging circle closer – meaning if when you started you were 20 feet from the trailer move the circle closer to the trailer so when your horse passes the door he/she is now 15 feet from the trailer. Remember this is just a suggestion of distance and each horse will have a different starting point. Listen to your horse. Look for the signs mentioned above and reset the beginning point if necessary. Finding the right spot will begin to build courage and confidence in your horse without introducing fear.

The next step is the most difficult for most people but if done calmly this will make all the difference in how your horse will load in the future. Once your horse can walk past the trailer opening without any sign of tension it is time to move him/her closer to the trailer.

Begin by lunging your horse past the trailer and then when you feel he/she is comfortable change your lunging circle to more of an oval than a circle. By that I mean when your horse is at the furthest point from the trailer and just at the point where his/her left eye (assuming you are lunging (counter clockwise) can see the trailer opening use your core (belly button) to move your horse towards the trailer opening. Remember horses are like a rear wheel drive machine so do not make the mistake most people do and pull your horse towards the trailer – drive him/her towards the trailer by positioning your body at your horse’s rump with your belly button aimed at your horse’s elbow and your horseman’s stick pointed towards  the ground but out behind your horse – remember you want your horse to approach the trailer at a walk not change gaits because YOU positioned yourself incorrectly or demanded too strongly.

As you move your horse closer to the trailer you are now a barrier to him/her escaping to the left by going between you and the trailer AND 100% of the time YOU are the weak link. Your horse cannot go to the other side because the open door is acting as a barrier so you are the easiest escape route. So BE AWARE! Do not position yourself so close to the trailer that you block the horse completely from escaping. We want to allow him/her to make a decision – hopefully the correct one.

If your horse chooses to run between you and the trailer door it can be used as a learning opportunity to teach your horse that this was not the correct decision to make. We want your horse to figure out what it is you are asking.
If this continues to happen you were not ready to ask him/her to move towards the trailer and you should go back to walking past the trailer on the lunge.

The next step is to ask your horse to get into the trailer.
One of three things usually happens:
A) The horse moves forward.
B) The horse backs up
C) The horse runs between you and the trailer.
If B or C happens you have not spent enough time getting your horse to go forward nor has your horse been given enough time to build courage to go between the trailer and the handler without changing gait or without changing frame of body.
Note: It is incorrect to believe that once you start your trailer loading lesson you MUST get your horse into the trailer. As long as you stop on a positive note you can continue the next day. In fact I know many horses that really benefit and build courage when they have a longer processing time.
If you do get A above and your horse moves forward it is important for you to relax, breathe, release all pressure and let your horse look inside the trailer. Then ask for another TRY by taking your horseman’s stick out behind your horse (pointed towards the ground) and move it towards his/her butt. This is important – Too many people add pressure by holding the stick too high and pointing it at the side of the horse and not behind. This pressure asks the horse to move his hip away from you and as you know horses work on a diagonal so if the hip moves away the head is coming towards you. – Not what we want! If your horse looks into the trailer or paws at the trailer as I said take the pressure off by taking the horseman’s stick away. Wait 20-30 seconds and then ask for more forward to actually put a foot on the ramp or up into the trailer. Reward any effort by removing any pressure, relaxing your body and praising the effort. The nice thing about horse training is you get lots of “DO OVERS” so if at some point your horse cannot take any more pressure and as a trainer you can read the body language make it your idea to abort the mission and take your horse away and begin the whole process again.

When you do get your horse to enter the trailer by using the method described above you then have become a person who Plays Horse Games by Horse Rules. You drove your horse into the trailer. You did not lead by the head which is a predator move nor did you put yourself in a dangerous position by entering the trailer ahead of your horse where you can become trapped very quickly.

AND you allowed the time for your horse to build courage and confidence.

As a further tool in training your horse to load if you cannot get him/her to stay focused at looking into the trailer but rather he/she is looking from side to side for an escape route attach your 22 foot rope to the halter and run it inside the trailer and out the front door or window and then back along the side of the trailer to the handler. So now the tail of the 22 foot rope is in the left hand of the handler and the horseman’s stick is in the right hand.
REMEMBER – The rope is NOT to pull the horse into the trailer but to block him/her from looking right or left.  You drive the rear of the horse forward as mentioned earlier. Every time your horse moves forward you MUST release the pressure on the halter (as long as his/her head remains straight ahead). As your horse moves forward  pick up the slack in the 22 foot rope. When your horse enters the trailer walk to the window or door where you passed the 22 foot rope out the trailer and let the rope drop inside the trailer and towards the back of the trailer. It is preferable at this point to have a friend standing at the trailer door to stop your horse from rushing out before you get back there. Your horse will most likely have turned around before you get to the back of the trailer so all you have to do is grab the lead rope and lead him/her out at a walk. Try to stop your horse once in a while to ensure he/she relaxes, listens to you and does not rush out. Repeat the whole exercise 3-5 times and then 2-3 times a day for a week, then you can begin to ask your horse to load  without passing the rope through the trailer window or door.

Begin to keep your horse in the trailer for longer lengths of time and when he/she can stand relaxed you can close the trailer doors.
Stay close to the trailer and continue to reassure him/her.You have now built COURAGE in your horse by taking the necessary time. You have built CONFIDENCE because there was no fear in the training method and you have built COMPETENCE by repeating the steps over several days.

You have PLAYED HORSE GAMES BY HORSE RULES and your horse will thank you.