Case reports also lower a journal's impact factor, so top of the line journals are unlikely to publish it. Consider attempting journals that have a track record of publishing case reports.
It helps if you know the journal beforehand because some journals want them to be really short and don't want a ton of extra info beyond the case presentation but generally it's broken down by intro- two to three paragraphs with general background info on the topic/why it's important, why it's important, and what your case adds to the literature. Then case presentation- basic HPI info, diagnostic workup, hospital course, outcome. Then short discussion comparing and contrasting with other papers or explaining findings.