as for clippers. i have never used them before. i would assume you justtake your time with it. have the clippers off and let the horse smell it. once he doesnt care rub it on his neck and by his ears. when he is ok with that then i would turn them on and just move around his ears and head and neck and get him used to the noise without touching them with it. then when they settle attempt to do for the actually clipping of the mane/fetlocks etc.
thats just what i would do...
I'm with kait but to add on..
The way I've desensitized horses to clippers is make sure they're fine with the idea of being rubbed with the clippers turned OFF down their neck, around their face (carefully) down their poll to the tip of their nose. Brushing along the halter..
Once they're fine with that, I switch it on at a distance and turn it off. Usually I pretend like it's not about the clippers and grab a brush and brush the horse, then switch the clippers on while I'm brushing. I turn them off once the horse calmly stands still. Then I leave them on longer and "brush" the horse on its shoulder with the body of the clipper (NOT the blades!) so the horse can feel the vibrations. Then I turn them off and give the horse time to think about how that felt and how he is still alive.
Then I start moving the clippers from the shoulder to the neck, then to the crest, then down the throat latch both OFF and ON. Then I hold the horse's head down and turn on the clippers with the blades pointing AWAY (in case the horse jerks, you don't want to clip him on his ears or his forehead) making sure the halter is loose enough to slide back but still ON the horse
and usually the first few times you do it, they shouldn't be on the cross ties. Then I'd simply make sure the horse's head was low enough to see what I am doing. Focus on trimming the bridle path without the horse feeling they need to resist.
The horse might pull his head up, so I will add pressure to bring it back down. The horse might back up, I add pressure to bring him forward. The horse might do this... do the opposite to bring them back to where you want. I usually aim to do the entire bridle path but it's a good idea to desensitize the horse (in your own way or as I did above) to the clippers before you do clip .
Good luck :)