The Microsoft Word "add-in" may already be installed when you install Zotero 7.0. To check this, and install the "add-in" if necessary, go to Zotero settings. Then click the "Cite" link on the right (see screenshot below).

1)    With your cursor, click where you want your citation to appear in the document.

2)    Select “Zotero” tab in MS Word.        
3)    Click “Add/Edit Citation.”

4)    Search for your citation using author name, keywords, or part of the title. Then select it.

5)    Hit "Enter." This will add the in-text citation to your document.

6)    If prompted, set preferences for your document’s citation style.

7)    Click “Add/Edit Bibliography.”

8)    Bibliography in your chosen style will be added to the END of your document.