Just grab it firmly at/under the front edge and yank upwards -- it simply snaps out. If it's like the hatchback you'll be surprised how light it is; it's nothing but shaped foam. Almost all of my weight was in the 2/3rds seat back and the humongous metal brackets.
Here's
an eHow article that may help you. It mentions steps I didn't have to do -- like removing kick plates and finisher trim -- so it may be geared more towards your sedan than the hatchback.
In the "credit where credit's due" department, I found my original inspiration:
two 2008 posts in the Nissan Forums by "Alx". Rather than building a box for the hump as I did, he angled his load floor (and attached it only at the front). But you'll notice the shape and general design of mine otherwise mimics what he did.
So, thank you Alx!
Cheers,
Rick