Was just interested in how you would go about that
First, to be able to "send" your horse anywhere you need to have the basics down of "lunging for respect". Such as, yielding the hindquarters, moving the forehand, and your horse always giving you "two eyes" when just standing still--unless otherwise asked. Once you have those down, you can practice lots of sending exercises to really help develop the relationship between you and your horse.
So set up some obstacles; anything really. You can start with the basics--sending your horse between you and the wall/rail. Helps to have your horse in a rope halter and long lead and a carrot stick. The key to this exercise is for your to not move AT ALL. Your horse will do all the work and this really helps getting your horse used to going into tight spaces without you having to lead them through; such as sending them into the trailer to load up. Anyways, stand about 15 from the rail and send your horse out into a circle around you by pointing in the air with the hand with the lead rope. Cluck, kiss, or whatever command you give to have them trot. No need to raise the whip yet because he should go off the lightest signal first. He should pass between you and the rail during these circles without much thought or concern about being that close to the rail. If he is, just keep going until he's relaxed.
Then decrease the circle to 10 feet or so; just slowly decrease the circle's size by walking towards the rail. Remember to pick up the slack in the line. If your horse becomes anxious, like before, just send him on until he's relaxed. Again, decrease the circle's size by walking closer to the rail and picking up the slack in the line. By now you might be about 5 ft from the rail and your horse might be tensing up, unsure about the tight squeeze. No worries. If he stops and doesn't want to pass between you and the rail, again point in the air with the hand that holds the lead. If he listens just by watching your hand, great! If not, go ahead and use a vocal command. If no response, tap the air with your whip. If he's not listening, don't be afraid to tap him on the butt. Doing this should "send" him between you and the rail. Once he crosses in front of you, he shouldn't go any farther past your outstretched arm that is pointing in the direction you wanted him to go. In order to get him to stop, crouch down, pull on the line, step towards the horse's butt with your outside foot, and swing your whip to his hind end, thus "stepping on his tail" and making him give you "two eyes." The point is for him to go no farther than your outstretched arm and for him to yield his hindquarters.
Now put the lead in your other hand and switch your whip into your other hand. Now point in the air to the direction of travel; the hand that is now holding the lead rope. Repeating the steps before, send him between you and the rail and having him yield his hindquarters before he passes your outstretched arm. Make sense? Do this over and over until he is no longer rushing. Preferably to the point where he will quietly walk between you and the wall.
You can practice this same exercise by sending him between two barrels or next to a trailer. With the trailer, send him along the sides of the trailer. Once he has that down, with the tailgate up, LUNGE him between you and the back of the trailer. With the tailgate up, he shouldn't be concerned that you're trying to make him go in the trailer.When he is going with that, "send" him back and forth between you and the closed trailer. Once he is confident and relaxed doing that, you can let down the ramp and lunge him over again. Like before, you're not asking him to go in the trailer, just to merely pass by it. When he's relaxed, then you can "send" him back and forth over the ramp. Eventually you can get the point where you can just send him up into the trailer with no problem.
I hope this helps and it isn't too long! This is also good for the OP to read.
Also! I am really into Clinton Anderson, which is where this method is from. :) Look him up!