This article will explain the process of Mirroring your laptop display on a connected second screen via a cable or Apple TV.
The laptop or iPad must be connected before continuing or the following options will not be available.
Step 1 - Click the Apple in the top left corner then choose System preferences
Step 2: Select "Displays"
Step 3: Open the Arrangements tab
Step 4: Within the "Arrangements tab" you can select "Mirror Displays"
The current configuration as shown above will treat the projected image as an additional screen. If you would like to maintain this configuration and display something on the screen you simply need to drag your presentation for example, to the right hand side or your laptop screen and it will appear on the projector display.
NOTE - IF "Mirror Displays" does not appear or you only see a single screen showing please check the physical connection to the Projector or Apple TV