A case study is a description of a specific project that shows the starting situation, the chosen solution and the measurable result. It differs from an ordinary testimonial in that it contains figures and context, so the reader can compare whether their situation is similar. A functional structure is simple: who the client is and in which industry, what problem they were solving and what they had tried before, what you did and why exactly that, what the results were and over what period, and what you would do differently today. The last point adds more credibility than any superlatives. Before publishing, obtain the client's written consent, including the scope of the data you may state; if they do not want to be named, use an anonymised description of the industry and company size. Case studies are also a strong source for citation in AI answers.
See also: Customer onboarding, Original data as a citation magnet, Results guarantee.