In order to change your gearbox oil, you will need the following:

8mm Allen Wrench
2 litres of Dexron III compatible automatic transmission oil
An oil catch can or similar container
T25 screwdriver
(optional) A friend for the first and last steps.

Our car is a year 2000 smart city coupe with 50,000 miles on the clock, and as far as we know... The original gearbox oil. The car should be up to operating temperature when you attempt the following procedure.

STEP 1 (T25 screwdriver):
Remove the rear panel bolts, two by the wheel arch front, three just under the tailgate and two under the rear valance. Pull back the panels and they should unclip. You may need a friend to help you remove them as they can be unwieldy. Place to one side carefully.

STEP 2:
Locate the two allen bolts, there's one on the side of the gearbox near to where the left hand drive shaft enters (looking from the rear) and one underneath near where the two sides of the housing join.

STEP 3 (Allen key):
Remove the TOP allen bolt first. This is VERY IMPORTANT as if you forget and do the bottom one first, you could be left with no way to get oil back into the gearbox, and you'll cry.

STEP 4 (Allen key and oil catch can):
Place the oil can under the gearbox and undo the bottom allen bolt, please try not to drop it in the oil otherwise you'll get very messy trying to fish it out again. Let the oil drain for about 5 mins, there's only 1.4 litres in there anyway.

STEP 5 (Allen key):
Replace the bottom allen bolt and tighten to 40Nm (or just fairly tight!).

STEP 6 (Dexron III oil):
Follow the instructions on the oil on how to open it, and place the oil spout into the top gearbox hole, fill the gearbox by  tipping the bottle up and squeezing it until the first one is empty. Do the same with the 2nd bottle of oil and stop filling when it starts to run out of the top hole. Your gearbox is now full!

STEP 7 (Allen key):
Replace the top allen bolt and tighten to 40Nm (or just fairly tight again!).

STEP 8:
Replace the rear panels, it's the reverse of how you took them off.

You don't NEED to remove the rear panels, but it does help when refilling the gearbox as you can actually see what you're doing. Do not jack up the car as you will not get all the oil out.

As ever, we do not take responsibility for any damage you cause to your car while attempting the above procedure. If you break it, it's your own fault.