There is no easy way from within an open instance of Chrome. You need to go to the terminal and open a new Chrome instance in application mode, passing the URL.
OSX
/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --app=URL
Windows
Chrome.exe --app=URL
(prepent Chrome.exe with the Chrome folder, which varies by Windows version - see here)
Linux
google-chrome --app=URL
Download my free Programming Ebooks!