Having decided on your learning outcomes, type of case, level of information given and overall structure - and whether the case is to be used for classroom discussion, an assignment, or an examination. At different levels of study (undergraduate, Masters, Executive Education) you will have to make a judgement about how accessible the case is to students.
But once you have got all of that sorted out all you need to do is to decide what to write about.
So...write about what you know.
In a Business School faculty members are divided into FOUR types, according to AACSB and as noted in an earlier BLOG entry:
|S = SCHOLARLY; I = INSTRUCTIONAL; A = ACADEMIC; P = PRACTITIONER|
So, all they need is a spark of imagination, the motivation to engage students and the comfort (for those that need it) that not every waking hour needs to be spent pursuing research to do your job fully.