First, make sure your horse fly mask is clean! Regular washing is necessary to prevent any bacteria from being transmitted to your horse's face. Be sure not to approach your horse from the front, especially if this is their first time wearing one and or your first time installing it on your horse. Start at the shoulder of your horse and if there is a Velcro strap, be sure to undo it before putting it on your horse while standing next to their shoulder. Your horse will probably turn its head toward you to see what that noise is and maybe even take a sniff, and that's perfectly fine! Before trying to put the fly mask on, rub it along your horse's shoulder and side until they see okay with the feeling of that material on their skin - this is just to make your horse more comfortable while you install the fly mask. Startup by the ears and then work your way down until it is completely fitted and feels correctly installed. You can now walk around the front of your horse to make sure it is snug and fitted correctly on your horse's head. You want to make sure that it is properly adjusted, Velcro (if any) is securely in place, and the ear coverings (if included) are correctly placed on your horse's ears.
Note: The Velcro should be snug, but not too tight that it will impact his or her movement or make him or her uncomfortable.