How can I add outlining to an existing document?

I have a document that I cut and pasted from a PDF that I have to add government-style outlining to. It has main paragraphs and sub-paragraphs down three or four levels. It is pretty easy to do outlining from scratch but I am having a great deal of difficultly figuring out how to add it.

The outlining features are based on the Heading styles.

So go through your document and apply the Heading styles to the appropriate paragraphs.

Heading style shortcuts:

Ctrl + 1 = Heading 1

Ctrl + 2 = Heading 2

Etc. …

Maybe superfluous: but it is not necessary to select text: just putting the cursor in the paragraph is fine.
And mind the difference between end of paragraph (Enter) and end of line (Shift+Enter)