I would urge you to use the built-in styles that come with Word. Note that you can modify these so they look the way you want.
Chapter = Heading 1
Title = Title or Heading 2
Subtitle = Subtitle or Heading 3
Why use Microsoft Word’s built-in heading styles? by Shauna Kelly

If you are starting a major project with Microsoft Word, you owe it to yourself to learn a little about the tools you are using.
Basic Concepts of Microsoft Word - from Shauna Kelly
Understanding Styles in Microsoft Word