If you followed through on the last three steps, you should now have a collection of papers relating to the plan for your website.
Congratulations -- you're well on your way to writing a complete design document.
A design document is simply a comprehensive plan for your project. If you want to do anything that is even mildly ambitious, a design document is strongly recommended.
A concise (one page) overview of the project that communicates the basic concept. What's it all about?
Motivation, intended use, importance. Why are you doing this? What will you do with it when it's done? Why does it matter? Articulate the standards by which the success of the project may be measured.
A logically organized, hierarchical outline of the project's content.
A chart depicting the user's experience -- how he or she can navigate through the content.
Nonfunctional, annotated sketches of key elements and screens.
An exhaustive, detailed list of all the media necessary for the project. Account for copyright issues, if any.
A detailed plan which spells out how the above will be accomplished. Who does what, and when? Include schedule and budget information.