I will probably write a book on this topic because there is not enough material in English. But the practice is fairly simple overall and only requires time and a positive attitude. Since there is a somatic response, it is always easy to tell if it is “working", especially for the "lower" exercises.

Originally, it had to be taught by MDs, according to Dr. Schultz--the inventor of the method--and his followers, but that ship has long sailed.

It is fairly easy to find a copy of Karl Rosa's book “Autogenic Training”, a good starting point, and Luthe's in-depth multi-volume analysis of autogenic training, though I would only recommend the latter to the most avid enthusiasts.