Summarize this article in one sentence.
It may seem self-evident, but knowledge is the key to writing a successful manual. For example, if you're writing a camera manual, knowing that an f-stop and the shutter speed are not just 2 separate functionsthey arebut knowing how they interact with each other will make it much easier for you to describe each function as it relates to the whole. If your role is simply the writer rather than the subject matter expert, involve knowledgeable people throughout the process and make sure they review your work. Their knowledge and advice is invaluable. If possible, doing the thing you are writing about will, at the very least, give you a feel for what the user will want to learn. Learn to talk the talk, and become conversant in the product you are writing about. Manuals of similar products will show you how other writers have tackled the subject. Look for similarities among the writers, which indicates both common functionality and common approaches to describing something. Look for differences that stand out. Those are likely to be functions that are unique to a given product. Your product may or may not include those functions, or may have alternate ways of solving the problem that you can describe, enhancing the value of your product. While your job may be writing a how-to, showing the customers the value of their purchase is a great way to encourage them to continue reading. Devour trade magazines. Find out how the people who use the products work with them day-to-day. They may wish there was a function that solved their particular problem, and if your product is the solution, that needs to be highlighted.

Summary:
This is key. Talk to experts. Try a hands-on approach. Read up on the subject.