Training horses always involves some risk of injury (or even death) to the horse, the trainer, any spectators, equipment used, etc. - this risk is totally your own as you will be making your own judgements on what to do. As with any suggestions on any subject you must weigh them out for yourself and proceed accordingly - at your own risk.
My suggestion is to first read the article one or more times and then watch one or more of the demonstration videos found after the article. I suspect that will help you understand best. But I know a good many will watch one or more of the videos first - just remember the article combined with the video will, in my opinion, help you understand best.
Many of the problems humans have with horses stem, in my opinion, from the fact that humans (even the vegetarians) see and react to the world through the eyes and the instincts of a predator while horses see and react to the world through the eyes and the instincts of a prey animal. Humans have an advantage - they can learn to see the world a bit more through the eyes of the horse - they can learn to stop talking predator body language to the horse that is instinctually afraid of predators - they can learn to speak horse body language to the horse - they can learn to reason with the horse so that the horse understands what is wanted, agrees it is safe for the horse to do, and willingly chooses to do it when asked.
Let's mentally look for a moment at the horse's natural environment - Steppe also known as High Plains - only hills (and tall grass) obscure the sight radius for a long ways. The horse does not want to get eaten by horse eating predators - and their instinctual defense is to stay far away from predators they see approaching OR to get far away from predators that have managed to get close to them. Horses not only look for potential predators they also look for ambush points - places that squeeze them in - places that limit potential escape routes. And of course they look for changes in the landscape pattern that might tip them off to a place a predator could be lurking in ambush.
Now let's mentally look at our horse and/or stock trailer. To the horse our horse or stock trailer looks something like a "cave on wheels." For the horse - they see just one way in and out and it is a narrow way in and out to boot - so it from the very start looks like a place that their anti-predatory instincts tell them they should avoid because it looks like an ambush point. And then once they do get inside this "cave on wheels" moves and all sorts of traffic noises and sights are experienced whilst they are quite literally trapped inside this "cave on wheels" rolling down the highway 55-70 miles per hour. Wow!! Sense that it is no small thing to get a horse to trailer calmly. Sense that is a a doable but BIG task to get a flight response prey animal to trust YOU enough to calmly get in your "cave on wheels" and calmly ride in it for sometimes many hours and then calmly unload once the trip is over and then calmly get back in again later on. Keep that in mind. YOU may know it is safe. The horse must learn it is safe despite its instincts telling it otherwise. It's instincts put the horse or stock trailer firmly on the Ambush Point list and your job is to reason with the horse until the horse figuratively pulls out its black Sharpie marker and crosses the horse or stock trailer off its list of Ambush Points it should stay away from - and instead puts it on its "tested out as a safe place" list. --- I would highly recommend that before you begin working on trailer skills with your horse that you first learn about helping horses overcome fear at https://griffes.fyi/overcomefear because you will need to help your horse overcome fear while teaching it to load and unload well from a horse or stock trailer.
Leading WELL is the key to getting the horse to calmly load and calmly unload from the horse or stock trailer. And candidly MOST horses I see do NOT lead well - and right now I hear a whole lot of horse people protesting - saying "I can lead my horse with a piece of string and it will never take the slack out of the string." To which I would say - that is a good and essential start. But, quite candidly, leading well means that the horse follows you as the herd leader ANYWHERE you go - including if you go somewhere that the horse is afraid of - because that horse that puts the brakes on hard and by horse body language says to YOU "I am NOT going there" - that same horse will follow the four-legged herd leader ANYWHERE it goes - it will trust that four-legged herd leaders judgment above its own judgment out of earned respect. So if your horse will not follow you ANYWHERE then let's face it - your leadership needs to improve so that you acquire earned respect and the horse does follow you anywhere it would follow a four-legged herd leader AND yes follow you places that four-legged herd leader would never lead and do it out of earned respect for you as the two-legged herd leader. So yes - leading WELL is the key to getting the horse to calmly load and calmly unload from the horse or stock trailer. Start there and remember that leading well includes backing well - because the horse does need to know how to back out of the trailer - backing is required for the horse to exit straight load trailers - backing is required to get out of the last slot of slant load trailers which have a back tack area - yes your horse needs to know how to back out even if you have a open stock trailer because somewhere in its future it may get hauled in a trailer that it must back out of - so teach it - prepare it. You never know when your horse may need to be trailered in a trailer other than your own - so get it ready to calmly be trailered in any trailer even one that requires backing out if yours does not require backing out.