While editing a document, press play or use the hotkeys to activate the reading. Or, use the Read Selection icon to only read from specific portions of the text.
While in 'View Only' mode, highlighting is deactivated but you can still follow along through the CC Panel in the Reading Bar.
Google Sheets and Google Forms
You can also read from Sheets or Forms while in 'Edit in Google Sheets/Forms' mode only. Use Read Selection to highlight and listen to the text.
The extension cannot read directly from Google Slides. Download the Slide to .pptx or .pdf and upload directly to NaturalReader Online to read from it. 'Read selection' is unavailable from the context menu (right-click) while using Google Docs.