Just did this the other day: The front (towards the front of the car) pulls up, and then the back has a tab that once you pop out the front, you slide the plastic thingy toward the front of the car.
Don't just pull up on the front and back of the piece or, as mentioned, you will break it.
The real PITA is getting the 4 "tabs" just below the vents to line up. The cheap cardboard used for the parcel shelf is annoying at times...