-a karate instructor should be at least a sandan (3rd), this is the minimum rank where you are eligible for the license of sensei, in japanese and okinawan organizations some require you to be a yondan.
-the most important quality of a good instructor is they must first be a good student. there is always something to learn, you should keep an open mind.
-and always have a lesson plan ready and be able to make changes to it as the class dictates. you never know when someone may ask you to teach something.
-have an understanding of human anatomy, and body mechanics.
you should also have an idea of some basic first aid and take a few classes in it.
-explain each technique and then demonstrate it.
-watch how your instructor teach and what he does, and how he organizes his class, dont try to be exactly like him be yourself, just follow his example.