Relatively few dog trainer programs are available, so finding qualified professionals to learn from is crucial. Look for a trainer willing to take you on as an apprentice. This is how most trainers learn the profession.
Apprentices watch their mentors teach classes, take notes, and gradually assist with larger and larger portions of class. This allows you to gradually learn the profession under the supervision of an experienced professional who can provide feedback and guidance throughout the process. Apprenticeships vary, but most last six months to a year or longer.