Install El Capitan in Virtual Box on a Mac

I'd like to have a virtual El Capitan running on my Mac so I can run certain software without fear of security problems. I've tried to install El Capitan in Virtual Box, but can't seem to figure out how. I've found all sorts of advice on the web, but it's mainly for installing OS X on Windows or Linux and doesn't work on a Mac. At least I can't get it to work. I have found some places that purport to show how to install a virtual OS X on a Mac, but they are either incomplete or contradictory.

Is there a FAQ someplace that will let me create a virtual Mac on my Mac? Doesn't have to be in Virtual Box.

Thanks.

--JT