If you have a halter with a very long lead rope (usually rope halters, we have one with a 10 foot lead so you can do groundwork) you can take the lead rope and swing it back and forth, when they are fine they do it over their back from one side to the other. Slowly they won’t care anymore. Then you can take objects like tissues and wave them around. Rub them with the object and give them treats so it isn’t associated with anything bad. You can work up to tarps, umbrellas, big scary things essentially. Some horses already have that temperament; my horse can have a tarp laying on her, can walk over a tarp, walk past an umbrella no problem. This isn’t something we worked on, she just didn’t care lol. Good luck and I hope this helps!