Four photos, about ten minutes. Easiest with a friend on the lead rope. No saddle or pad on, and pick level ground with good light. See the tutorial video on this page for a detailed walkthrough.
Stand back far enough to get the whole horse in frame, from the tip of the nose to the tail. Horse standing square with all four feet under them, not resting a hip. Level ground.
Same shot from the other side. Standing square again, whole horse in frame.
Stand on a bucket at your horse's head and shoot straight back down his neck and withers. Hold the camera level and don't tip it down toward the horse's back.
Stand on a bucket behind him and shoot forward from the base of the tail. Camera level again, ears in the frame at the far end.