Guidelines for authors are generally listed on journals' websites. If you have a journal in mind, check their guidelines and requirements before writing. There are other organizations that have reporting guidelines for different study designs. For case reports, review the following:
CARE Case Reports Guidelines
Developed by the CARE Group in 2013, the CARE guidelines provide a framework that supports transparency and accuracy in the publication of case reports and the reporting of information from patient encounters.
Other resources are available to assist you in learning about case reports:
A Guide to Writing and Using Case Reports
This thematic series, published in 2016, provides a valuable resource for clinicians who are considered producing a case report. It is comprised of a special editorial series of guides on writing, reviewing and using case reports.
Medical Writing: A Guide for Clinicians, Educators, and Researchers, 3rd ed. (2018)
Chapter 7 covers case reports.
The recent history of the clinical case report: a narrative review
Nissen T, Wynn R. The recent history of the clinical case report: a narrative review. JRSM Short Rep. 2012;3(12):1–5. doi:10.1258/shorts.2012.012046