How do I insert citations into my document using Mendeley?

Use the Mendeley cite add-in for Word, you can download it from here.

Mendeley Cite will download the reference metadata from your Mendeley cloud library and you can add citations directly from the add-on – you do not need to have Mendeley Desktop open or even installed, just make sure your library is synced to the cloud, then select the references you want to cite and click 'insert citation' to add the citations where your cursor is.