After your Pages are made, return to the one you will use to link to your Path.
Click the pencil icon to edit the page and look for the "Relationships" tab below the Page body section.

From this drop down menu, select "Path."
The blue number next to this option indicates the number of Pages linked by Paths, so yours should be empty if you're just starting one.
Click the "Add more content" button to add pages or media to the Path. You may drag these to reorder them. You also have the option to have the end of the Path or direct the reader to another page.

A Contents section will appear on the Page. It contains an index of the Pages in the Path as well as a button to follow it.

Readers can continue through the Path by clicking the buttons at the bottom of each Page. Above the title of the current Page is the title of the Page that contains it, as well as its place in the current Path.
Clicking this title will bring readers to the Page.

Next to the button to continue on the Path is a smaller, grey button with a left pointing arrow. This allows readers to navigate back.

Just as set up when creating the Path, the end directs readers to the top page.

The final part of this guide will provide examples of how to create more complicated Paths to connect Pages in your Book.