Macropod (Paul) is trying to tell you that this will not be easy.

First: work with a copy of your subject document. Some things may be difficult to undo.

Second: attach a template containing your styles to the document. This can be done using the Developer Tab's Document Template button. Check the button to update styles from the template. Then go back to the Document Template button and uncheck that checkbox.

Third: examine at your document. What, if anything has changed already, with just this step?

There are three kinds of styles, paragraph, character, and linked. Find the text you want to apply the style to. For instance, if I wanted to put bold text in a style, it would be the character style of "Strong." You can do a replace changing text with the font formatting of "Bold" and applying the Style "Strong."

I generally simply start with a plain text document (no formatting) in a template that has the styles I want. If then apply my Body Text style to the entire document and go through and apply other styles as appropriate. That gives me the best result but it can be a lot of work.

Another approach is to open (a copy of) your document and then apply AutoFormat to it. This will not apply styles, other than headings, though.