1) Word is not illustrator. For manuals, it is probably better, but it is a different program.
2) Create a template and base your manual on that template.

• Templates in Microsoft Word

3) Learn to use Styles.

• Understanding Styles in Microsoft Word

4) If you are going to have numbered items or headings, connect your numbering to styles.

• How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly

5) Learn to use the StyleRef Field in headers and footers.
6) Learn to use Content Controls. If the same thing is to be repeated multiple times in a manual, consider using the Document Property Content Controls.

• Word Content Controls
• Repeating Data Using Document Property Content Controls and Other Mapped Content Controls

7) Learn to use Building Blocks and store some just for your manuals in your template.

• Automated Boilerplate Using Microsoft Word
• Building Blocks & AutoText

What you've asked contemplates quite a bit of learning on your part. The time you spend on that will be recovered quickly both by you and by anyone using your documents.