Being understandable to everyone
The report is actually the trickiest part of engagement. You may have found the slickest, the smartest, the most impactful vulnerabilities of your whole career on a device, but if you are not able to deliver your message in a clear and understandable way, finding that and nothing is exactly the same... Let's see how we can minimize the risk of being misunderstood by the client.
Building your report template
The first fundamental thing is: use a template. Not only is reinventing the wheel for every report a waste of time but just imagine yourself in the client's shoes. If they receive two entirely different documents, with different structures and a different approaches, they will have a lot of trouble understanding the point. And if this is a re-test (very common after you have found problems and they want to ensure that the vulnerabilities are actually covered correctly), they won't be able to compare the reports to understand...