Dash Floor Vent Installation

So there was always something sort of loose behind the radio and center of the dash. Since I was vacuuming I took a look down there and noticed that something wasn't attached. Pulled it out and googled the part number and it was a dash floor vent. I tried googling it a bit more to find out where exactly it's supposed to go but can't find anything.

TL;DR - Where and how to I reinstall my dash floor vent?
(Picture isn't mine but exactly the same part)