Help with installing a sub box under the rear bench in my f350?

The truck is a 2006 f350 King Ranch Lariat with a crew cab. I'm trying to keep this as cheap as possible so im doing it myself. Im looking at this box on ebay that goes below the back seat. The item number is 260801739574. I know i have to take out the seats, but I dont know where to go from there. any help would be appreciated