You can download the latest version of Grammarly for Microsoft Office at http://www.grammarly.com/office-addin and follow the instructions on that page to install it.
To learn more about how Grammarly for Microsoft Office works and to make sure that Grammarly is compatible with your computer, please check this article.
Note: If you use macOS, please check this article for more information about Grammarly for Microsoft Word on Mac.