How can one become a holistic health practitioner?

It depends: It depends what type of holistic practitioner you choose to become. Homeopathic, holistic medicine, integrative medicine, herbalist, naturopath. Once you are more clear, you can choose a path. Good luck to you!

Be convincing. : If people think you are helping them they will come to you. You just need people who will pay you to help them. Then you can offer cures and regimens that they are convinced will help and rationalize away any failures You really don't need any specific training as many of the suggested regimens are unproven and don't hold up to scientific rigor.