Jason Feifer created this blog that collects selfies of people at funerals. Here are the pics that were collected in only two days.
That's quite simple, you just need to remove the polarized layer from your computer screen and use new polarized sheets to make polarized glasses out of 3D glasses for example.
Only the person wearing the special glasses can see what's on screen, others will just see a blank monitor.
Better try that first on a used monitor you don't care about though...