At the bottom left of the main video feed, you'll find the "screen sharing" icon, which looks like this:
By clicking this button, you can choose whether you want to share your entire screen, just one application, or even a specific tab in Chrome.
This is what your screen should look like when sharing:
To stop sharing your screen, click the blue "Stop sharing" button at any time.
Note: Users running MacOS Catalina (10.15) or later will need to follow the below steps to allow screen recording:
Go to System Preferences,
Then choose Security and Privacy:
Click on Screen Recording and mark the checkbox next to Google Chrome:
Screen sharing will now be enabled.