The first step in customizing the layout or formatting of a structure application is to create your own application; the easiest way to create your own is to clone the existing apps.
First, copy the FrameMaker\Structure\XML\DITA-FMx folder (with all subfolders included) to a new folder name, for our purposes here, we’ll call it DITA-MyCo. In the new DITA-MyCo folder are the three application folders (Topic, Map, and Book), and the DTD folder. You’ll be modifying files in the application folders, and leaving the DTD folder as it is (unless you’re specializing, which we won’t go into here). You’ll want to make the same initial changes to each of the applications (replace “<app>” with the Topic, Map, or Book application in the steps below).
Once you have followed those steps for each application, you’ll have created a “clone” of the default applications and are now ready to customize them as needed.