You can install the Narratives or Tableau Chrome Extension from the Google Chrome Web Store. When you reach the listing for the product, you will see a page that looks like the below:
To install, click the blue "+ Add to Chrome" button at the upper-right of the listing. You will be asked to confirm the installation.
Click "Add Extension" to complete the installation. You will see the extension icon show up in your Chrome toolbar and you will receive a confirmation pop up.
Once the extension has been installed, you will receive feature updates automatically and you can use the extension when working in Tableau worksheets or dashboards.