Documentation Strategies?

An anonymous reader asks: "I'm a developer and have been given a task of creating documentation, for fellow developers, on how the system (a CMS) I have implemented, and adapted, works and how to develop for it. On the surface it doesn't seem too complicated but the amount of information I need to get down on paper, and the level of detail needed on some parts, is great. What's the best way to approach this task when there's so much information bouncing about in your head you don't know where to start?"