Open the required file in viewer on which watching needs to be started or stopped.

Next, follow the steps for navigating to File Viewer More Options.

In the 'More Options' menu, select 'Start Watching' / 'Stop Watching' option (under 'More Info' category) to start / stop keeping a watch of activities performed on the file revision which is open in viewer. 'Stop Watching' option will only appear for files in which 'Start Watching' is already enabled.

Clicking on the option will display a prompt message if the selected operation was successful.

Click here to understand how to configure watch settings once you have started watching a file.