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How to teach horses to step on a tarp in hand
Pin the tarp to the ground in advance.
Secure it as tightly as you can in an area protected from wind, but within sight of the horse's exercise area. The less the tarp blows around, the better.
Ideally, give the animal a few days to get used to this " scary monster " before you continue. Before you continue, wait for a calm day when there is almost no wind to blow the tarp.