A Good Grounding
Part 16: Tarps
Please read and heed Rules of Engagement before trying this exercise.
It isn't uncommon for horses to find tarps to be very scary. They are big, they move, and they're noisy; this is exactly the combination of characteristics that a large prey animal such as a horse is hard-wired to avoid.
Place a tarp on the ground, then practice the Sending exercise near it. Don't ask your horse to touch it or step on it at first, just work him near it until he relaxes and stops worrying about it. Once this happens, you can move slowly closer to the tarp as you send him. When he gets within reach, he may want to stop and sniff, bite, and/or paw at the tarp. Let him! This evidence of curiousity is just what you are looking for. After he has had a chance to check it out, send him past it again, getting closer all the time until he steps on or over it. At first he may jump over it. Don't worry about it. Just keep sending him back and forth over the tarp until he relaxes and walks calmly over it.
You can do a similar exercise with a tarp hung on a fence. This is especially good on a windy day.