No matter how your book gets published, sooner or later you’ll have to be involved with marketing it. If you have a publisher, they may or may not engage in some initial marketing activities. After that, you’re on your own!
Marketing your book has a dual purpose and only one of them concerns selling the book. The other purpose is to tell people about the book. The reality is this: the day your book became available, thousands of other books also became available. Since you are an unknown author (so far!) no one knows about your book and no one cares that the book is available. This is the situation in which marketing comes into play. You have to tell the world that 1) your book exists and 2) make them care enough that they investigate the book and possibly purchase a copy. Without this marketing effort, your book will languish in the realm of the unknown.
In this phase of your book project, I provide more advice and less templates than in the other phases. That’s because marketing a book (or anything else) is a vastly complicated procedure that doesn’t lend itself to filling out a few simple templates. However, these templates will establish the basics and produce an author platform you can build upon.