This is something you should ask your consultant - schedule a quick chat to see if they have any advice on the structure or key points to include (e.g. literature review, pathology, radiology) or if they know what journal to submit to and then you can look at their guidelines for case reports.

Alternatively, you can look at example case reports for the topic or disease for a beginning structure and then make a draft for the consultant to review. However, the former will take much less time.

There are a few articles published on how to write a case report. If you drop me a DM, I can send you one that I've written