Before you embark on creating a new piece of documentation you should survey the existing documentation, both official and unofficial. If there is already a piece of official document that has roughly the same topic and scope, then it probably isn't a good idea to create a new artifact. Once you determine that your project isn't redundant, then you should survey existing material with an eye to incorporating as much of it as possible into your project. The more content you can harvest from existing writing, the better. An outline is a hierarchical breakdown of a topic that describes the structure of the artifact.
Outlines are the most important organizational tool for all actors involved in documentation.
Authors use an outline to guide the content as it is written. Editors use an outline to evaluate the topic, scope, quality, and progress of an artifact. An outline consists of a sequence of top-level, key ideas that encompass the scope of your topic. Each top-level key idea should be broken down into a second level of more specific details of each key idea. In necessary, an outline can descend through as many levels of detail as are needed to describe the topic.