Hold her lead rope in your left hand about 18" from her halter. Then start spraying toward her upper legs/shoulders. If she moves, fine. Just keep moving with her. The INSTANT she stops, stop spraying, drop a loop of the lead, and take a step back. Give her a minute or two to think this through. Then try again. You may be spraying as she runs in circles for 20 minutes if need be. But you stop when she stops. Her circling will get shorter and shorter, and eventually you will be able to walk up, spray once or twice, then step back and she won't move. That's when you're done for the day. Rinse and repeat. You may start over the second day. Keep at it. She'll figure it out eventually.
Only when you can walk up, halter her, toss her lead over her neck or hold it loosely and spray everywhere should you ever tie her for this.
I start with a spray bottle filled with water, then add some vinegar. Some horses are fine for just water, but react differently once they smell something odd. The vinegar seems to help without wasting fly spray, or add a small amount of spray to the water. Then with the first bottle of actual fly spray, if need be you can add a few glugs of vinegar to the bottle so it's not so different. Most horses are reacting to the sound of the sprayer and the feel of the mist, but some react to the scent. Also, be aware that just because a horse stands for fly spray from a squirt bottle does not mean he will stand for spray from an aerosol can. One guy had a wreck at a show this Sunday with that-- his wife bought an aerosol can of spray because the store was out of the regular, and his 3 y.o. colt apparently had never experienced the Horse Eating Spray Can. The horse was caught in a residential area 3 blocks away one broken halter, dented trailer, cut leg, missing shoe, and broken fence later. Luckily the local vet's daughter was also showing, so he just went to his truck, got his emergency kit, and sewed up the colt right there. The horse will be ok, but it was a scary situation and could have ended very differently.