Unless you're working in an organization that has specific documentation standards with which you must comply, the ultimate structure of your document depends on its scope and your own taste. It can be as simple or as complex as you think is appropriate; there is no standard format. I'll give you some guidelines in this chapter, but I'm doing so on the assumption that you'll adapt them to your specific needs.
If you've followed the recommendations I've made regarding the system analysis, you'll find that the document easily falls into several well-defined sections that (not entirely accidentally) coincide with the previous chapters in Part III. You'll probably also want to include an Introduction or Executive Summary aimed at management.