I recently inherited a Coronet Captain box camera. Where could I find a manual or some instructions on its use?
Online, your simplest and easiest options are to google on the name of the camera and "user manual", or to find out if the manufacturer still exists and has a website and look through their support/downloads sections and/or to contact them directly. Or you may want to start scouring eBay. Ephemera, such as manuals, are often sold to collectors as well as the cameras themselves.
There are also websites like this one, maintained by Michael Butkus Jr., who collects vintage camera manuals and makes them available online as PDFs. The suggested donation is basically to help him feed his collection. Butkus has a PDF for Coronet box cameras here: