Do not expect overnight success by any method. Firstly, position your trailer in your horse's field for at least some of the time. This will allow your horse to become comfortable in its presence. At this stage do not force your horse to go to the trailer, and instead wait until he can go past it or next to it without noticing it. The next step is to try and feed your horse next to the trailer and then on the ramp and eventually inside it. At all times your horse must be allowed to leave the trailer area whenever he wishes. The important lesson here is to allow your horse into the trailer for something enjoyable and let him leave whenever he chooses - this will instil confidence.
Once your horse is prepared to spend time in the trailer in a relaxed fashion, attempt to shut the trailer behind him. If he becomes stressed then open it again. Continue trying this until he can spend time in the trailer when it is shut (but not moving). Once your horse is comfortable with this step, you can begin to slowly move the trailer while your horse is in it.