lolwut?
It literally is a button you press on the main Steam window (View>Big Picture; or rightclick the systray icon and select "Big Picture Mode") It also can go into BPM by pressing your connected controller's home button so you don't even need a mouse and keyboard. There's no setup aside from installing Steam, does not require any intimate knowledge of Windows, and just turns the interface into something easily navigated with a controller while also being scaled for a larger display sitting further away.