Even though your manual will say the autobox capacity is 9ltrs, you will only be able to drain out about 4.5 to 5 ltrs, the rest stays in the Torque Converter (TC).
On a TDi 300 the oil change is dead easy.
Warm up the oil by going for a short drive, take out the gearbox dipstick and leave to one side, then get underneath.
Bearing in mind you will be collecting up to 5ltrs of old oil, place a suitable sized container under the allen key sump plug. When you are ready undo the sump plug and let it pour. Leave for at least half an hour to get out as much as poss (good time for a coffee) then replace the sump plug.
Assuming that you are NOT going to change the filter screen we carry on. Examine the old oil - what colour is it, red or brown, clear or thick - smell it, does it smell burnt?
Now we start to refill the gearbox, with Dexron IID fluid and you'll need max 5ltrs (probably about 4.5). The filling is done via the dipstick tube.
Put in about 4ltrs, then start the engine and with hand and footbrake on, select 1 and wait five secs, then select D and wait, then select R and wait 5 secs, then select P. Leave engine running.
Go back to the engine bay and using the dipstick, check the level. It may not be high enough to register on the stick, if it doesn't put in 0.25ltr then check again. Do this until you have a level between the two marks on the 'stick.
Between low and high is only 0.25ltr so go carefully on the last bit of filling up as over-filling is bad.
So, that's step 1 !!!
Step 2 is do it all again in 100 - 200 miles !!! This ensures (as best is possible on home mechanics) that most of the 9 ltrs has been changed.