What you need:
-A gallon of G12 or G12+ coolant. I believe our Audis need non-glycol based coolants, which can be found at the dealer for about $25 for a gallon.
-Some way to get your bellypan off your car (rhinoramps or car jack)
-A gallon of distilled water.
-A hose (connect to a water supply)
-Big bucket or catch pan
-Large flathead screwdriver
First take off your bellypan, there are 3 screws near the bumper, two towards the back of the pan, and two hidden plastic screws in the wheel wheels. Everything can be taken off with a quarter.
To get to the drain plug you need to take out the grill panels from your lower bumper (the ones to the left of the drivers side fog light). There is a slot for a screwdriver but just pull hard and they pop off. You will see a big grey flap of rubber. It will be stiff to work with but bend the top left corner of the rubber down until you can see a big red (+) plug. Unscrew the cap from your coolant fill tank and set up your catch bucket. Using the screwdriver you can unscrew the red plug if you use a weird angle. Unscrew it until the coolant really starts to pour.
Now using the hose run some water into the coolant fill tank. Tighten up the red screw and fill the tank up. Cap the tank and turn on your car with the heater on full blast. After a few minutes you will see the water get sucked down into the radiator, so open the cap and top it off. When it stops sucking water in shut off the engine and redrain from the red plug. This can be done as many times as you want, but it should be done at least twice.
Refill the tank with a 50/50 or higher concentration of coolant to distilled water. Again be sure to run the engine so that you fill up enough.
Coolant is posion, be sure to dispose of all the liquids properly.
When draining an S4 we used the huge coolant line which was on the passengers side, underneath the front end. This was harder to work with but drained everything in 5 seconds