How to to familiarise the horse with all that frightens him. Faverot de Kerbrecht

Progression to follow to familiarise the horse with all that frightens him. – To habituate a horse to the noise of drums, of music, and to flags, etc., it is in within the silence of the school, and not outside, that the rider must give the lesson. He must follow an extensive gradation. For the drum for example, the lesson begins with several strikes of the stick. As soon as the horse is disturbed, stop the noise, calm him down, flatter him, start over, etc. Give this lesson with the drummer at the halt at the beginning, and then have the drummer march towards the horse. Everything in progression. For gunfire, hold him far away at the beginning, even very far away, and then approach the shooter almost imperceptibly.
Should the rider happen upon an object that the horse fears, he must guard against trying to approach the object with vigour. On the contrary, let the horse be as far away as he wants from the subject of his fright, and give (the reins) completely. Use the rein and leg as little as possible. Pass and re-pass the object many times, far away to start, and then closer and closer as the horse pays less attention to the object that frightened him in the beginning. To achieve his goal more safely and more quickly, the rider should begin by giving in hand every lesson intended to tame a horse, to reassure him, and to familiarise him with that which worries him or frightens him. -Faverot de Kerbrecht available here