Once you have added some citations to your document you can create a reference list. Place your cursor at the end of your document where you would like your reference list to start and go to the Mendeley Cite add-in window (via the button the Word toolbar).
Select the ellipsis button (3 dots), menu, and select 'Insert Bibliography'.
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The bibliography will be formatted according to the selected citation style.
Each time you add a citation or make changes to your citations, the bibliography will restructure itself accordingly. For example, if you are using a numbered style and you choose to insert new citations early in the document, the bibliography will also re-number itself.