My method changes depending on the time and the length of his coat.
During the winter, when he's longer coated, I'll take some scissors, and cut some of the length off to make it easier on my clippers, since my clippers I use for this aren't exactly built for cutting through long, thick hair.
From the fetlock down to the coronet band, I'll go against the hair.
From the fetlock and up, I'll go down with the hair, since it's easier to blend.
Around the coronet band, I start at the very bottom of the coronet band, and get all the little hairs that can get long over the hoof.
The key with a winter coated horse, is to not completely clip his legs of all the hair. So you're going to leave a little more hair than you would then during the summer.
During the summer, I'll just go against the hair the whole way, and then around the knee, blend it out.
It is best to clip when the horse is clean. For legs, you can just get a damp towel, and rub the dirt out, and then towel dry. Make sure his legs are completely dry. Wet hair is no good on clippers either.
"You're just as sane as I am."~Luna Lovegood.