Sorry, I din't read any of the above replies. I'm too impatient, lol, & I'll lose my train of thought.
The "SCARIEST" object any horse I've ever worked with is a plastic food store bag, filled with empty soda cans. It makes lots of noise. I also use tarps, and lunging whips/carrot sticks.
While the horse is tied, bring whatever object you decide to use up to the horse. Let her smell it. Touch it. Play with it. Whatever she wants to do with it. Touch it to her sides, back, neck. Don't go behind the hip just yet, if she spooks you might get kicked.
Then take her out to an inclosed space. With a lead on her, make the objects make noise, throw it around her. Then rub the object on her again. Put it on the ground, walk her by it, trot her by it. Do this on both sides of her.
Then with the tarp, spread it on the ground. Let her paw at it, sniff it, etc. Then, with you facing her, walk backwards, coaxing her to walk on to it. If she walks, pet her & coo, be very rewarding. Let her eat grass even. Walk her over it both ways, then try to trot her over it. She may not right away.
If you really want to try for her to not mind the tarp, hang it on a fence. Let it blow in the wind.. Free lunge her by it. She may spook hard, but din't reprimand her for it.
Hope this helps.