Certainly, this is the most daunting task of the whole process. You've asked the questions, done the preliminary work, made a few mockups, and now it's just you with a blank Word document (or Framemaker, an XML editor, or whatever your preference may be) and a head full of ideas. Hopefully though, it's not totally blank. Throughout the first five steps, you've ideally been taking notes, developed a working outline, and maybe even started writing a few sections as they come up. Although each writer will invariably have their own style, I would like to pass on a few general tips that you may find useful in structuring your document:

• Cover everything
  By this, I mean make sure you've written about every single interaction point that exists in the application. Don't leave anything to guess or assume. It's quite likely that you'll come to a point where you believe you have included everything, only to find you have left out something very obvious. Every link, text field, button, etc. should have a detailed description of what it does and how the user experiences it.

• Use lots of screen shots
  It's usually helpful to slice up your mockups and insert screenshots of the individual elements you may be writing about. And always include a screen shot of the entire page at the end of its section.

• Write concisely, correctly, and consistently
  An economy of words will be appreciated by those who have to read your document and make sense of it all. Additionally, try and break things out into logical components or steps as much as possible. By writing correctly and consistently, I mean make sure you're using proper grammar and consistent terms (e.g., if you use "drop down menu" in one part of your document, don't start using "pull down menu" in another part to mean the same thing).
• Use the tools and format most comfortable for you
  As a spec writer, you still have quite a few decisions make. What tools do you use? What format? What sort of style should you adopt? Etc. Also, you need to take into account how your documents will be used: will they need to be online, will they frequently be merged with other documents (e.g., large technical libraries), do they need to be in a particular format or follow a certain style? Really, there is no single correct path for all of this, and it depends on you: the writer. I have frequently used Word, PhotoShop, and Visio to create my specs. This setup has its pros and cons, but I have also found myself in a position where I needed to adopt the system of my employer…or one of their clients, etc. This has led me down the DocBook/XML path, the PageMaker path, and the FrameMaker path. Like I said, it really just depends on a number of variables; the most important being what you prefer and how your documents will be used.