For large projects I write documentation similar to how one would write a book or article. I start with an outline of the big picture. Next I fill in the details. After that I ask peers to provide feedback. I do one final round of updates based on the feedback. This helps discover any missing information or areas I can improve the organization of the documentation. Once that is done I release the info onto a wiki. When new people are added to the project they will always have questions about the documentation. A wiki makes it easy for the new people to add to the documentation once their questions are answered. A wiki also makes it easy to update the documentation when it becomes out of date.