Most horse in general don't like the smell of fly spray. But Put a good halter on her with a 6 ft lead rope and take her and your spray bottle to the round pen and go to work. Start out with water, then you could do a water/vinegar mixture (for smell) then move to your fly spray. But I just start spraying them and let them lunge around me, when they decide to stop running around I stop spraying. It takes time but they figure out that it is not the acid rain they thought it was.
Or make sure she has a fly mask for her face and don't spray her at all.

Another tip some horse don't like the sound of the spray bottle or areosol can. You could dry putting your fly spray in a hand held weed sprayer that you pump up. It is quieter and goes on more evenly. Just make sure it is a new sprayer.