Zotero's word processor plugins are available for Microsoft Word, LibreOffice, and Google Docs. Microsoft Word plugin is installed by default, and optionally for LibreOffice if you have that installed on your computer. Google Docs support is part of the Zotero Connector browser plugin. Third-party plugins are available for other word processors (e.g. LaTeX). Read instructions for Microsoft Word below or see Zotero documentation for Word, Google Docs and other word processors.
When you install Zotero, Microsoft Word plugin is also installed in the package. Add in-text citations in your document and create bibliographies automatically with the reference style of your choosing. The plugin is available on the upper menu in Word, and you don't have to log in to it separately.
Image: Zotero plugin in Word menu. Click to enlarge.
Add in-text citations by clicking the "Add/Edit Citation" (if Zotero isn't running, the plugin tells you to open it).
Next you will be asked to choose a citation style. You can change the style afterwards, and add new styles from Zotero (Preferences -> Cite tab: Styles -> Get additional styles).
Start typing the title or author to the citation dialog that opened. The dialog box suggests references that match your search. Choose the reference you wanted and press Enter.
Image: Adding a citation in Word. Click to enlarge.
Edit an in-text citation by choosing the citation in the text and click "Add/Edit Citation". Choose the citation in the dialog box, and you can e.g. add page numbers to it, or suppress the author. Click Enter when you are finished.
Image: Editing a citation in Word. Click to enlarge.
Add a bibliography based on the in-text citations you have used in your document, by choosing "Add/Edit Bibliography".
If you add new in-text citations, the bibliography is updated automatically. If you remove a citation, you have to update the bibliography manually: click "Refresh".
Image: Zotero bibliography in Word. Click to enlarge.