Make sure your car is turned off. You do not need to unplug your battery.
Open your driver side door and remove the trim using the trim removal tool that was provided in your package. It will pop right off very easily.
Remove the two bolt pictured here so that you can remove the panel below the steering wheel.
Remove the four bolts on the underside holding the panel below your steering wheel. Then pull the panel off. There are a couple tabs on either side of the steering wheel that will really hold on to it. The couple cars I've done I had to really yank it off, nothing broke, but be careful once it comes undone to disconnect the headlights switch otherwise you run the risk of breaking that wire. Once you have this panel removed set it aside.
You need to remove the leather cover on the flip down DVD cover. It's really easy, just open it and grab it as pictured and pull the face only towards you.
Remove this single bolt that holds the trim around the DVD.
Pop off the left and right side of the trim around the DVD. There are more clips around the bottom, but you may not need to take it all the way off.
Place the ends of the screen cable as shown so that it can reach to the right side (left for RHD) underneath the steering wheel where we will leave it dangling for now.
Now place the two tongues on the bottom of the screen into the grooves as shown, tighten the cable slack and push the screen into place while gently spreading the sides of the leather trim out a little to allow the plastic hooks on either side to behind the leather trim on either side. Once the screen is in place, push the leather trim clips back in.
Connect the other end of the flat cable now to the port on the end of the box according the photo
Find a suitable place under your dash to securely mount the both with zip ties or tape.
Plug in the power cord into the power port as shown on the image above.
Now since you opted for an A8/S8, you have the luxury of NOT having a switched 12v easily accessible like all the other models. If you have already ran a flex fuel sensor then you already know about this and the good news is you should be able to piggy back off that switched 12v you rant for it. If not, this gentleman kindly did a nice write up of exactly how to get to it. You need to run the power and ground wires accordingly. Alternatively, some have tapped two of the pins in the obd, but I don't support this method.
Optionally, plug in the provided USB extension cord to one of the blue USB connectors and run the other end to somewhere you can access after driving to download your logs to the provided USB stick. I've provided a velcro sticker so you can place the USB plus somewhere on your lower dash if you like.
Test, when you turn on your car ignition, your bTres should come on after 30-60 seconds. If it comes on, you are likely done and ready to put everything back and enjoy. You may have some codes that you'll need to reset using bTres or ds1 web interface afterwards.