There many ways to approach a case study and it depends on the purpose First find out why you are doing it? what can you learn or teach with it?
Make sure you know where your placing this case study prior to creating it. Make sure it is a case study the site/users will be interested in. I like to go on sites, and look for questions on a specific topic or a trending topic within my niche. Only then do I create a case study. Once I have what there going to want.
Research, look up everything regarding the topic. Cover all grounds, use anything factual to support your case.
Keep track of what your doing from day 1. I like to use quicktime video screen recording for this. Most people just take screenshots. I open a Wordpress document from day 1 and I add as I go. This helps me keep track of everything I am doing. Write your case study day by day, and at the end of this project you will have a unique pice to publish.
Most important part of a case study is getting answers to questions that no one has solved yet. Before submitting a case study, make sure it is complete. Did you answer any questions? is this enough detail for the readers to learn? can I add anything else?
If your answers are: YES | YES |NO
Submit it and you have a great case study.