During the warm up, or the first session, have the horse soft and loose and in front of your leg. Then let him have a stretch so that the oxygen can go round to his muscles The warm up will depend on the time of year, whether your horse has been stood in the field and how fit he is but ten minutes is a good guide.
The second session is when you pick your horse up into a test outline and work him, normally for about 20 minutes.
Then spend the third session, about five minutes, cooling down, making sure he is relaxed and stretching.