When you open your Google Calendar and click on the event listed, you will see something like this:
Click on the link with the green video icon to bring you to Google Hangouts.
Click on the green "Join" button to be part of the call where you will also see Andrew's face and hear his voice (and he will see your face and hear your voice).
At some point, Andrew may ask you to share your screen.
To do so, click the green "Screenshare" button.
Now click on the option to share your "Entire screen."