You can take an online training course, learn by doing it yourself or participate in a practical course. However, due to the lack of regulation in the industry, there are many people who call themselves dog trainers (or animal behaviorists) with no education or experience whatsoever. As a professional, certified dog trainer myself with 10 yo of experience, I find experience to be the most important thing when choosing a trainer. You might know dog training principles in writing, but doing it is completely different.