this can be a tricky process. open the frontdoor to your trailer so the light shines in and it's more welcoming. don't work alone, you need at least one other expiriened person, perhaps your coach. there are many different ways to do this. grab a lunge line and put it behind your horse. apply a little pressure to encourage him to move forward. usually a horse would sit on it then pop forward. if your horse does not take to this method, try getting the person working with you to grab a lungewhip and tickle their heels (sounds weird, i know) but it'll make them uncomfortable and move forward to get away. i love this method because it doesn't hurt them and it's about getting the horse to do what he wants to do. if your horse does not take to this method either, gently & repeatedly swing a lead rip on your horses side, a soon as he steps forward take the pressure off. give him a reward. then continue, one step at a time. however, if you're horse won't even go near the trailer, start by grazing him next to it and walking him by it, do this to get him comfortable with the trailer. once you can get within 3 meters of the trailer and remains calm and comfortable, you can you begin those steps. there are plenty of other methods you can use if these don't work for you. i wish you the best of luck!
What I did is I backed the trailer up into a barn or pasture so only the ramp and back of the trailer is blocked by the sides