Complex sentence: Ideally, the horse will be desensitized by careful training to tolerate a gentle stream of warm water into the sheath from a hose or a large, needleless syringe.
Split into simpler sentences: Ideally, the horse will be desensitized by careful training to tolerate the procedure. To begin, a gentle stream of warm water is run into the sheath from a hose or a large, needleless syringe.