The process is easy. If you don't have it, download Fluid. Open Fluid, and create a new app for URL http://www.npr.org/tablet/, and name it NPR or something else. If you want to, you may want to Google for a high-resolution copy of the NPR logo, so that you can use that as the icon for the app. For that, in Fluid choose Other in the Icon pop-up menu.
After the app is created, launch it, open the application's preferences, select the Behavior section, and make sure the box next to 'Closing the last browser window' ('only hides the window') is checked. That's it.

