Remove the false floor (two washers holding it in by the rear wall).
Remove Floor carpet
Remove Rubber trunk grommet going around trunk opening
Remove Drivers side Luggage fasteners
Remove Drivers side panel cover (snaps off, just take your time around the trunk arm)
Using the supplied spacers and hardware, set the tank down over the battery tie down and use an extension and possibly a swivel to use the longer bolts and mount the tank to the floor using the battery tiedown holes.
Using the supplied airline cutters cut the airline from the factory tank (leave enough straight section to insert the supplied T connector). You can do one of two things, Remove the airline from the tank to slowly bleed the air out of the tank, or use the cutters and cut the line. If you cut the line make sure to not jump when you cut it as it will be loud. The D4 kit will retain the factory tank AND the new tank. Use a pliers or something to hold the airline from falling back into the grommet. Attach the T connector. Match sure the airline is pushed into the T connector as far as it will go.
Run the supplied Airline from the T connector down to the Tank
Poke a hole in the carpet roughly where the tank is to connect to the tank.
Reinstall all your trim in reverse order!
You will need to drive your car around to fill up the factory tanks, the compressor won't kick on while the cars idling to fill up the tanks, So refrain from airing out for a least a days worth of driving before seeing the full benefit of the tank install!