To train a dressage horse, learn the basic walk, trot and canter, work on longitudinal control, and then focus on lateral work. Build on basic movements to work on more advanced dressage training with help from an equestrian team head coach in this video on horse training.
Training a dressage horse is a very systematic approach to riding. So, to begin with, you want to make sure that you have the basics, walk, trot, and canter. Once you have the basic gaits you're going to start to work on longitudinal control, extending the gait, and collecting, and then you'll begin working on lateral work. We'll start with leg yielding. Once that is easy for you you can move on to shoulder in and haunches in. These are the basic elements of training a dressage horse, and from there you can build with more advanced movements.
Specialty: Hunter, Jumper, Equitation