Connect using an HDMI cable directly to the Chromebook or connect using a VGA to HDMI converter
Newer chromebook will need a USB-C to HDMI converter
You can also toggle between Mirrored and Extended Desktop modes from the Chrome OS with a keyboard shortcut. Just press the Ctrl & ( F4 ) keys at the same time.
If you chose Extended Desktop mode, you can drag and drop the displays to control how they’re oriented.