Simply go to the Zotero downloads page, click on the download button on the left side of the page (you'll focus on the Connector part in the next step). Follow the instructions for your device.
After downloading and installing the Zotero app, you'll want to set up the Connector This piece allows you to quickly download citations directly from library databases.
Using the Chrome or Edge browsers:
Return to the Zotero download page to install the Zotero Connector.
OR 
Using the Safari browser:
The Zotero connector for Safari is bundled and installed with the app, HOWEVER, you must turn the connector "on." To do this,
Under the "Safari" menu, choose "Settings."
Choose the "Extensions" panel, and check the box next to the Zotero connector.
