A: Approach her real gently, all quiet-like. Stroke her neck softy. Whisper into her ear. Tell her it won't hurt a bit. That it's for the best. That she'll grow to love it.
Give her a sugar cube.
Show her the bridle. Let her smell it, touch it, get comfortable with it.
Make large, slow movements. Nothing sudden. Nothing jumpy.
Slide the bridle on, slow as can be. Maintain eye contact the whole time. Once it's all the way on, fasten the snap, stoke her neck one last time and give her a kiss on the lips.
Hand the reigns over to the stranger who's about to mount her, collect the money and start the camera.
That's how it works with my wife. I figure it should be about the same with a horse.