How do you desensitize a horse to a rain slicker or jacket that the rider puts on while mounted?

A rider putting on or taking off a rain slicker or jacket while mounted is a surprisingly common trigger for horse accidents, because the slicker's visual movement, its crinkle noise, and the fact that it suddenly appears above and beside the horse from an unexpected direction all combine into a highly alarming stimulus for a horse that has not been prepared. Clinton Anderson addresses the slicker specifically in his training program as a practical safety exercise that every trail and working horse should go through. His desensitization sequence begins on the ground — rubbing the folded slicker on the horse's body from nose to hindquarters, allowing it to make noise near the horse, then unfolding and refolding it while the horse watches. The ground work continues until the horse stands completely indifferent to the slicker being moved around its entire body, including being swung over its back and draped across its hindquarters. From the saddle, Anderson begins with the slicker folded and tied to the saddle so the horse can smell and see it while working. He then holds the slicker in one hand while riding with the other, allowing it to hang and move naturally. Next he opens it slowly beside and above the horse — moving it in the same crinkle-producing way a rider would when actually putting it on — before putting his arm into one sleeve while mounted and pausing before putting his arm into the other. The sequence is slow and deliberate specifically because the moment riders put on slickers is almost always urgent — it has started raining, or they are cold, and they want the slicker on quickly. Practicing the motion unhurriedly while the horse is calm builds the habit that prevents the accident that happens when a rider forgets to prepare and puts the slicker on urgently in the rain.