Unfortunately there are NO courses/instruction to teach credentialing.. and if there were or you came across them, I'd have to question them. I think this is a service that you can provide that really is based on experience and knowledge of the industry as a whole. There are also various tiers to credentialing that many overlook when offering it to a potential client. Some offer straight credentialing, but if you have the experience and know how you can take it to the next level which includes fee-schedule negotiation, contract negotiation, etc.