When I started clipping I was using my cordless Wahl Cordless Pro with a 10 blade:
Clippers - 10 or 15 blade for most clips - a 30 blade is too close
Well lit area, preferably with cross ties
Clipper brush, oil, blade wash/cooler if you feel fancy
A dry, clean (preferably freshly bathed) horse
Tape or chalk if you are worried about getting lines correct
Approximately two hours of free time
I like to give a bath the day before and make sure the horse is nice and clean, and dry. A wet horse dulls your clipper blades - and so does a dirty one. I usually spray the horse liberally down with a shine spray before hand to make the hair optimally easy for clipping. I find that using body markers works best and ends up looking nice and symmetrical- meaning that I measure distances using horse's body marks. This works especially well for doing necks and throat areas.
The clips I do are either a modified irish, or bib clip that goes up higher than this picture:
But doesn't clip the shoulder, which I like to leave hairy for blanket rub reasons.
Last year I ended up doing a somewhat whimsical interpretation of the below, low trace clip. It got kind of out of hand...
When clipping, always go with the grain of the hair. Hips and whorls are hard, because the hair goes in all directions, and I think that's why my hip clips got a little crazy. Go slow, give yourself plenty of time, and remember, you can always leave a job partially done - you could even be nice and make sure the blanket covers the undone spots so your horse's friends don't laugh at him. I always try to wear something slick that hair will slide off of, and good light really helps to keep you from getting too close and inhaling your horse's hair. Velcro, or wearing a polar fleece jacket is a bad idea if you ever want it to be clean again...
Also, remember that you are taking your horse's natural blanket away from him when you clip (even if it does make him more comfortable!) and have a selection of blankets on hand to keep him warm in all conditions. Foxie didn't get clipped until November, and I try to resist touching up in January or February. She went on a coat supplement (Smartpak's Smartshine) in the late winter as a shedding helper, and I have never had her shed out funny, or felt that it messed with her coat. If your horse starts coating up in early October, I'd give him a few weeks to get the coat started, then clip, and let the rest of the coat grow in, so you don't take too much off, but still get the relief of clipping.
See, not too scary, huh?
Ashley & Fox