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Dirt and debris in the horse's coat will dull your clippers as you use them, so it is best to groom your horse well prior to clipping. When possible, give your horse a bath the night before you plan to clip to remove as much of the dirt as possible. Use chalk or masking tape to section off the areas of your horse's coat that you intend on clipping. Make sure to use straight lines and mark off all areas before starting to remove hair. The buzzing noise put out by clippers is frightening to most horses, especially those who have never been clipped before. Let the horse see the clippers, and then turn them on a few feet away from its face. Allow them to see that the source of the noise is from the clippers. Alongside the sound, the way the vibration of the clippers feels on the horses skin might spook them. Test out their reaction by turning on the clippers, and placing the handle-end on their side. This will allow them to feel the vibrations without removing any hair. If your horse spooks particularly easy, place your hand on their side and the handle of the clippers on the back of your hand. The vibrations will travel through your hand and they'll be able to feel it on their skin indirectly.
Groom your horse. Mark off the areas you plan on clipping. Make sure your horse is not frightened of the sound of the clippers. Get your horse used to the feel of the vibrations.