How to properly document a project?

[MasterGod]

My project is stating to gain some meat and I'm afraid I wont be able to easily maintain it as I can now because I'm not properly documenting it.
Can you suggest me of ways to properly document a (big) project?

[rogerborg]

• Concept document: if that changes by much, then you're really working on a new project.
  Architectural document: list the tools and 3rd party libraries that you rely on, including versions and where to get them. Provide an overview of the main components in your system, and how they should act. You don't need an entire work flow diagram for the whole system, just an outline of what components rely on each other, and a general idea of the states that the system can be in.
  User stories: they, in toto, describe your entire product.
  Test plan: this should flow from the user stories. If you don't have a test plan, then you won't know when you're done.