Body Clipping 101 -
Start with a clean horse and a fresh pair of clipper blades.
Have blade wash, oil, a brush for the clippers and clean towels readily on hand.
Wear old clothes and eye protection (safety glasses or something similiar)
Clip from the tail and go forward, clipping against the hair direction. Clip the legs last, and consider skipping them if your horse is fussy or the hair on them is light. Brush off the clippers, dip in blade wash and oil often. Check the temperature of the blades against your hand often.
Anything that looks rough or uneven, run the clippers over again.
Consider having a friend with a pair of clippers come and do one side while you do the other. Goes faster, easier on the horse and then you have someone to hold a foreleg out while you clip the girth area, bend the head and neck, etc.
The horse will appreciate being hot toweled or rinsed off after the clip.
That's the big stuff. The difference between a good clip and a bad one is usually about two weeks, not that it matters since you're mostly doing this for the horse's comfort.
PS - I know several people who clip Cushing's horses in the summer, I clipped my Appy pony in April because of the heat.