Learn about the activities or sports you will be teaching. When I get the opportunity to design a program to train someone for a sport, I first interview the client about the activities they do. I watch him or her do the movements used in the sport. After this evaluation and perhaps some research, I design a routine to improve the sports movements and to prevent muscle imbalances and injuries. Acquire technical and strategic knowledge that is used to coach athletes.
All good instructors know how to simplify and teach complex movement.
Excellent instructors are skilled at observing movements and breaking them down into the base moves. They teach the student with a simple to complex learning curve. Most group instructors do not spend a lot of time teaching muscle anatomy and exercise science. The use of correct terminology with a visual location of where a muscle is can help the students understand the exercises. I like informal explanations when I'm teaching health club classes.