Flies make a horse's life miserable in the summer, and a fly spray is a necessity. Always be sure to follow label instructions carefully. Also, some bulk products may require dilution and the label will give the proper amount.
Sprays can be applied all over the horse's body except on the face and head where I recommend using roll-ons or wipe-ons. Get the horse used to the noise by spritzing a sprayer filled with water during your horse's dinnertime. Spray at regular intervals away from the horse until the horse just ignores the sound and continues to eat. Then try spraying while in the stall, being sure to stand close to the horse and spray away from it, near his shoulder, or totally out of kicking range. Next, acclimatize the horse by spraying the shoulder with water, not the neck, which may make him jump or the legs which may make him kick out. Continue at a light level then increase pressure of the spray, moving on to other parts of the body once he accepts it at each site. If he objects, return to his comfort zone and then try again. You can also get a pump sprayer that requires priming and will produce the mist quietly if your horse is touchy. Remember to reapply if it rains or the horse is sweating a great deal.