Some MSCs offer sample reports as part of their training information. Take a look at the FAQs and other resources the various ones offer. Different companies have different requirements, so if they don't have examples the best you can do is pay attention to feedback editors may send you. Writing reports for mystery shopping is a different experience than almost any other form of writing. You need to provide fact, not opinion. You need to provide specifics. Frequently a rewording of the question is how you need to respond.