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'.
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.