How do you get a new blank document on Word 2013 to hold normal spacing between lines of text. Not 1.15 or 1.5 lines, but just 1.0?

In a new, blank document, which you can create by pressing Ctrl+N, press Ctrl+Alt+Shift+S to display the Styles pane. Click the Manage Styles icon. On the Set Defaults tab, change the Line Spacing to "Single," select "New documents based on this template" and then click OK.

Later, if/when you are prompted to save the Normal template, choose to do so.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

Stefan has told you how to do what you want. Occasionally an Add-In may interfere, so if it doesn't stick, write back.