I start out with clippers like I do any other object; I will start with getting the horse used to the sound being around him, but not touching him...both sides, starting fairly far away, and getting closer as he gets used to it. I retreat only when he is relaxed (ie, feet not moving, head lowers, licking lips, cocking a hind foot, etc).
Once he is used to it being around and near to him, I would start desensitizing him to them touching him, but not with the blade, just the body of the clipper itself while it runs, again, retreating only when he relaxes for that part of his body. What you do on one side, you always do on the other side, so repeat that area on the other side. I start back on their shoulder area. I also may stop there for the first day, especially if it's a fairly nervous horse. And the next day, start with desensitizing the air around him again, and then starting where you left off with touching him with the clipper.
Some horses, like my current mare will easily get used to it...it took her about a half hour, from desensitizing the air around her, to me being able to trim her bridle path...others may take days to get to that point, so its up to you to determine 'how much/how far' in one session is enough for your horse.
I also prefer a cordless clipper, to a corded, as it's easier to let the horse move his feet that way; I DO NOT tie a horse I am trying to desensitize to a clipper (or anything for that matter)...if you take away their ability to move their feet, you make them more clausterphobic in a situation that they would rather move about. I prefer to let a horse move his feet, because it takes the "fight or flight" out of the situation for the most part.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."