Jack the car up on all 4 corners and place car on jack stand

Note: Typically the transmission filler and drain plug are so tough to remove you will need to get creative and use a combination of tools to assist you in loosening the plugs.  I do not recommend using just an allen key, it can easily strip the plugs as it is soft aluminum.  I basically used a 3/8 high quality 10mm allen socket with a breaker bar and a long extension attached with a socket to insert into the end of the breaker bar to give me leverage. 

 

Remove the underside panel

Place a oil collection pan under the transmission

Unbolt the transmission filler plug with a 10mm allen socket

Unbolt the transmission drain plug with a 10mm allen socket and drain fluid

Install the transmission drain plug and torque to 22 ft lb

You can fill the transmission fluid using several methods.  In my DIY I used a generic transmission fluid hand pump attached to a jug filled with transmission fluid, or you can simply buy a lengthy section of clear vinyl tubing from Home Depot and insert it into the filler plug and run the tube through the rear wheel and pour in the fluids that way.

Install the transmission filler plug and torque to 22 ft lb

Reinstall the underside panel.

Unlike other manual transmissions you just fill the fluid up until it starts to come out of the filler plug, the 996 transmission does not like the fluids to be filled to the brim.  So please follow what the factory spec. calls for and measure the amount of fluids to pour in.

According to the factory shop manual:

Carrera transmission filling capacity is: 2.7 litres, 3.3 litres for GT3