It is fairly simple to make an herbal poultice for horses. First you will need to gather the necessary herbs. The herbs you use will depend on the ailment you are trying to treat. Grind up the herbs in a blender then add a small amount of oil, vinegar or water to the herb mixture, just enough to make a paste like substance. Once the substance is pasty, spread the mixture into a soft cloth. The cloth should be big enough to cover the infected area. Apply the paste side of the cloth directly onto the affected area. It is advisable to wrap a bandage around the poultice to keep it in place, this is especially important when applying on horses. The poultice should be kept on the affected area until the mixture dries; this can be anywhere from a few hours to a day. Continue application until you receive the desired effect.