BMJ Case Reports
The Medical Library has a subscription to BMJ Case Reports, a good place to review how other reports have been written. Also, a helpful article from BMJ Case Reports is "Writing and publishing a useful and interesting Case Report.."
CARE Guidelines
Follow the CARE guidelines for writing Case Reports. The CARE guidelines (CAse REports) were developed by an international group of experts to support an increase in the accuracy, transparency, and usefulness of case reports. There is a useful checklist included, and the guidelines have been endorsed by multiple medical journals.