Everything I've ever learned about writing case studies for design projects, I've gleaned from Cook's Illustrated.
Every recipe that magazine/show produces is 100% food porn, following an explicit formula that yields not only deliciously readable prose, but also an understanding of what working process the chef/recipe optimizer went through. Much like a taking part in a design project, they're identifying problems, churning through failure after failure to find that most delicious outcome for you, the reader, to enjoy.
We designers may be spending more time in our kitchens making web apps and identity systems—which aren't as simple as, say, making a great crème brulée—but there are certain ingredients we can steal from the articles of CI to put that extra oomph into how we promote our design efforts to prospective clients.
Here's a taste of what I mean, broken up into the five key areas that comprise a great long-form design case study. Video and brief pictorial case studies are their own challenge, which I'll talk about at a later date. In this post, I'm describing those double-sided sheets of paper you may hand a client in a meeting to take home and peruse at their leisure, or more detailed case studies on your website. Some may argue that these kinds of case studies have gone the way of the dodo, but if you're working to be hired by a large organization with a long and elaborate sales process, you need a few case studies to pass along to all those people you don't get a chance to talk to you along the way.