Do they feel the topic is worthy of a case report?
It is not uncommon for journals to require an Attending Physician to be listed on the case report, so you will need their support for publication.
If there are already multiple case reports on the same topic, it is not worth your time to write and try to publish the report.
What is the background information for the diagnosis / treatment / etc?
Almost all journals require proof of patient consent, and some have their own forms that need to be filled out and submitted
Generally speaking, a case report on a singular patient does not need IRB approval, but if you're looking at a case series you'll have to be familiar with the IRB approval process.
Case reports should be written to be descriptive, but concise - they are usually only a page, or maybe two, long.
You can start with a brief introduction (one or two paragraphs) that covers what is novel about your case. You can present your case in a way that contrasts to what is normally seen / treatment that is usually applied. You can also mention any related literature here, as a mini-literature review. If doing so, try to keep it to 2 - 5 references.
Next, you'll want to report your case details. Again, stick only to facts directly relevant to the case. You can report de-identified patient information, how the patient presented, any testing/labs done, diagnosis, and treatment.
Finally, you'll want to write a brief conclusion or discussion. This should be only a paragraph that sums up the case and discusses what was learned.
*Depending on the journal submission criteria, you may be asked to provide other sections, like an abstract or the patient's perspective.
Go over the CARE Guidelines checklist to make sure everything is in order