See the Understand the Structure of Texts guide
Open your text
Find the section of text for which you want to create a child
Note: click on the section title in "Contents" to navigate to that section
Click on the plus sign
This plus sign is located under the section title
The "Create Book page" window will open
Type the new section's title in the "Title" field
Type the new section's content in the "Content" box
To add metadata to the section:
Note: typically metadata should only be used for the root node of a text
Click Metadata on the left sidebar
The "Metadata" page will open
Add authors, publication years, descriptions, and search terms if necessary
Click Book outline
Book outline is located on the left sidebar
The "Book outline" page will open
Verify "Book" and "Parent item"
"Book" should be the title of the text to which the new section belongs
"Parent item" should be the section you chose in step 2, the parent section
Note: see Understand the Structure of Texts for more information
Click on the "Weight" drop-down menu
"Weight" options will expand
Choose the importance of your section relative to sibling sections
Note: sections with higher weights will be listed first
Click Save
The window will close
Your section will be added to the parent page