I've just gotten a new horse, and as he's grown a winter coat, I think it's time to introduce our electrogroom, but to be honest, I have no idea how to. My last two horses came from the barn and were already used to it.
Set the Vac off to one side. This prevents horse damaging it if they get stupid. Hose is long, so you should still be able to reach horse. We use the tool that looks like an upholstry head for most grooming, so that is what I start with. Let horse check it out, hose too, while having someone hold him with a lead. When horse is done sniffing the tool and hose turn on one of the motors for some noise. I might even lead horse over to try getting him to sniff the vac itself while running.
Then have horse holder center horse in the aisle, stand him, and you can approach horse again with hose, tool on the end for more sniffing. I would be careful to NOT let the tool suck his nose skin! I start by putting tool on horse neck so I am nearer the front end in case I need to move away quickly. Horse holder is supposed to try to keep horse standing in place, OK if he moves head and neck, but not feet. Just start rubbing the tool over his skin. You move yourself and arms slowly, just like vacuuming a piece of furniture. Move on down his neck to chest, back to barrel, rump and haunches, stomach. Then back up, have horse holder turn horse around, start over the same way, do his other side.
Take your time, most horses seem to enjoy being rubbed on their big fleshy areas with a firm, not hard, pressure. If he is still silly, you may want to make this first session rather short.
I would repeat this for a couple days, but judge how horse is reacting to the hose and noise. If you think he will behave, you can move him to one side being clipped on a cross tie, horse holder keeping him steady and unmoving. Do that a couple days, while he shows you he can be reliable while sweeping him. Then move to using both sides of cross ties with reliable standing still. If he is still a bit dancy, you could try one cross tie, lead rope in YOUR hand for corrections while sweeping him. Move up towards a bit of ground tie training, instead of total restraint, so he doesn't hurt himself.
In my experience most horses LOVE being swept with vac. They like you leaning into them with pressure on the tool, lean back at you. I would expect you could turn on the second motor after the first couple sessions, to get the full cleaning effect, and horse wouldn't have problem.
We do a lot of blowing with our vac, so you have to move the hose to the other exhaust outlet, put on the pointed nozzle for blowing. Horse should be able to be blown off after several days of being vacummed regularly. Do have the doors open so the dust cloud will go away fast! Our horses like that blowing too, seems to feel like a gentle massage on them. Keep the nozzle from blowing on the nostrils, eyes and ears, under the tail or privates. Blowing does move a lot of dirt off the skin if hair is really thick and long.
We avoid bony part of horse wih the metal tool, legs, knees, skull, so they don't get bumped with it. Nozzle blowing does a good job of cleaning off mud if you use the hand, rubber curry to break mud up before blowing it off.
I bet he will like the vac in short order, not give you much trouble using it to clean him. I use it on the weanlings, yearlings, other young animals who stand held quite well for being blow dried after playing in muddy fields. They accept it readily.
I've done this a couple ways...
Do one horse who's good with it in front of the newbie. Then do newbie. Sometimes that's all it takes.
Rub newbie vigorously all over with it off. Then on low setting. Then progress.
I've seen someone progress so tentatively I think she made her horse nervous. Then she was told to just start using it on him briskly as if if was a non-event before he had too much time to get worked up. He was fine.
I brought my horse out of her stall with the vacuum in the aisle. Held her with a lead rope and let her sniff the grooming end. Turned the vacuum on to gauge her response and she was fine with it. Started to vacuum her and she loved it.
If the noise may be a factor, try earplugs.
She is not the quietest horse and won't let me close with a measuring stick!
I have a couple of nervous nellies that I thought would hate the electro groom. My cross ties are right by their stalls, so they got to see me use it on the one horse that was accustomed to it (and loved it). Several days of watching him enjoying it seemed to do the trick as both were surprisingly unconcerned when I did finally introduce them.
I did enlist a holder, feeling that s safer than cross tying, and did as others suggested, running the brush/nozzle over the with it off. The thing both were most worried about seemed to be the evil hose, and didn't take long for them to be ok with it. Then working with one motor on, I'd vac them in an "easy" area like the shoulder. And continue elsewhere as they relaxed...it really didn't take much at all. And then they all seem to like it once they get used to it, so now everyone is vac-broke around here...except the dog!