When the rider feels that control has been demonstrated and the horse is focused on the cow, The horse and rider push the cow around the corner of the arena down the long fence (on either side of the arena). Ideally the horse and rider keep the cow close to the fence past the middle marker (half way to the end) rating the cow to keep it moving toward the far end of the arena.
After passing the middle marker, before reaching the end marker, the horse and rider must change the cows direction 180° heading the cow toward the opposite end of the arena. This is usually done with a great deal of speed.
The horse and rider again turn the cow heading in the original direction. The rider may elect to make additional turns but only one in each direction is required.
The cow is taken to the middle of the arena and "circled" 360° in each direction. After the cow is circled in one direction, the horse and rider pass behind the cow to change the cows circle to the opposite direction.
The judge blows a wistle to signal the work complete after the circles have been finished in each direction. There is no time limit, but the work must be completed before the cow is exhausted. (NRCHA rules, breed specific rules may vary).
While the description is pretty straight forward and simple. The execution is another matter. The cow has a mind of its own and has no idea what you want it to do, nor does it care. The fence work portion of a reined cow horse show is by far the most complex and most unpredictable.