BILL— In a word: “judiciously.” And never in anger. It should be a reinforcement of your natural aids, the leg and the weight. They should precede the whip by a fraction of a second so your horse understands what you’re reinforcing.
It should be used lightly at first and progressively with more authority if it doesn’t produce a reaction.
It should not be used over and over in a nagging manner which only dulls your horse to its effect.
The primary place you apply it is close behind your lower leg, although there are occasions where use on the shoulder is necessary.
When not in actual use it should rest quietly against your upper thigh held in place by a mild torquing of your wrist. At those times it should not be touching the horse’s side.
Which side do you carry it on? There’s no rule. Carry it where you need it. Also get comfortable enough with it that it’s not distracting to have it in your hand. Better to have it right there when you need it than to stop everything and go looking for it.
Also learn how to switch it correctly [See this link: https://woodsdressage.com/q-show-me-how-to-switch-the-whip-the-right-way/ ]
Finally, if you’re competing, be sure your whip is of a legal length—47.2 inches. Don’t get yourself eliminated because the lash is a couple of inches too long.