A sheet of very clean glass is placed at a 45 degree angle between the set and the camera. 

Off to the side, and reflected in the glass, is the artwork you wish to superimpose over the top of your set. That artwork may be as simple as a piece of white paper! 

Depending on how much you light that piece of paper it will be reflected over the top of your set. 

Below we see the light dimmed on the set, revealing more of the actual artwork reflection used to give the illusion of light rays.