Once again for you lefties out there, here is a very good video explaining about the magic word "pronation" for a left hander on the middle and wide serve.
Ian Westerman shows his left handed serve and explains how to do it.
Some coaches think that you should not use the word pronation and just say "turn the forearm" whatever your pupil understands - it can also be explained in terms of your thumb on the racket hand- start and the thumb is pointing back and on the finish the thumb goes forward.