Once you have downloaded and launched the Mendeley Cite add-in for Microsoft Word, you can begin using Mendeley to cite right from the "References" tab. A new section will appear on the right side of the bar with the Mendeley logo that says "Cite with Mendeley". The first time you use the add-in, a pop-up window will appear asking you to sign in to Mendeley. Once you do, the add-in will open as a window on the right side of the screen with all of your references that you can sort or search through.
To add a citation...
To edit your citation style click on the "Citation Settings" tab.
To add a bibliography, click the three dots on the left side of the Mendeley Menu and then select "Insert Bibliography." You will need to have citations inserted in your document using Mendeley for this to work. This should generate a bibliography with the sources you've cited so far. As you continue adding citations, the bibliography should automatically update. If not, you can click the Refresh button in the Mendeley menu.