The process:

"The System" Installation

Sub Enclosure in the 911:

(see 964/993 installation on page for detail on those enclosure panels)

Plan plenty of time to get used to the whole setup and take your time getting a good "fit".  When I mount the enclosure panel in a Carrera, I leave only the factory mats in place, shift them around to get them settled on the floor.  Check beneath them to make sure the floor isn't dented upwards on either floor back there, then I push, wiggle, squeeze, and employ my knees, elbows, and fist to "work" the enclosure panel back as far as it'll go.   Then I drive the screws in the bottom of one side (through the hinges) at an angle which helps pull the enclosure in and rearward, nice and tight.  Then I'll make sure the other side is also seated, drive screws in the same fashion, then focus on the top brackets.  With a knee on the top of the enclosure, holding it against that bulkhead area, I'll drive a screw through the top bracket into the sheet metal under that carpet under the rear seat pads.  Sometimes if you don't get the right angle, the screw will just thread itself into carpet or foam, and you have to aim it a bit towards the rear to catch the sheet metal beneath that carpet.  You'll know when you have sheet metal because even a small gap will close and seal up very well along the top edge-even further than you thought it would go.  I use a Makita screw gun to drive all of these screws, and sometimes I have to kick that baby into low... 
When wiring the subwoofers, the positive terminal on each subwoofer is mounted to a red tab that's riveted to the subwoofer's metal frame.  The negative terminal is mounted on a black tab.  The 12 gauge speaker wire I recommend has a series of + signs on the positive lead, so just match polarity correctly in this step and the rest is downhill.  Give the female spade connectors that you're sliding onto the amplifier teminals a gentle squeeze with pliers so that they make a firm connection to the subwoofer.

This picture shows the supplied stuffing material.  Screws are supplied.  A cordless drill with a phillips bit really makes driving the screws in easy, just be firm and take good aim.  If you find that the screw is not becoming tight, back it out and aim for a different angle so you can drive it into sheet metal for a secure mount.  Make sure you don't shoot screws through the speaker wire, or any other harnesses in your car.  Lift those rear mats first, check everything and find the best fit.

This is what it should look like.  Only the factory floormats, properly positioned, remain under the mounted enclosure.  I've also mounted the amplifier on top of the passenger rear mat, this helps keep the amplifier cool rather than have a mat laying on top of it.   You might want to vacuum first!