I notice that no-ones come up with a photo guide so I will try to describe the procedure as it's not rocket science - even I can do it!
First of all you will need a very short length of 12mm hex bar, about 20mm should do - and a 12mm A/F spanner.
A tray to collect the oil and a 2 litre plus storage container (I use the larger size Co-op milk containers for both, one with the big flat side cut out to within about 20mm of the edge as the drain tray, - leave the cap on.
Some old cloths or lots of kitchen roll.
Rubber gloves to keep the gear oil off your hands.
Go for a run in the car to get the oil up to normal temperature.
Park it on a level surface
Jack up the nearside front and put a brick or similar each side of the opposite rear wheel to anchor the car.
Remove the nearside front wheel and pop it under the body sill in case the jack collapses or better still I recommend that you use an axle stand, under the body jacking point, not on the suspension.
Look under the car from the front and you should see to the right on the gearbox face about half way up a round plug with a 12mm hex recess. This is the oil level plug. Clean thoroughly all around it and remove using the hex bar and spanner.
Now move a bit further under and where the inboard gearbox casing meets the clutch bell housing follow the flat bottom towards the rear of the car, past the 2 bolt heads and you should see another plug similar to the level plug. This is the drain plug and it's awkward to get at, being so close to the suspension body support. This is why you can't use a standard hex key etc. on this one.
Clean around it and pop the hex bar into the recess and slacken it about half a turn only (hand tight) with the spanner.
Grab your drain tray and some cloth and unscrew the plug slowly by hand with the tray lined up underneath, it's a lot of twists but don't relax your grip 'cos thats when it will decide to drop out completely and squirt 2 litres of unpleasant smelling oil on to you and the driveway!
When you reach the end of the thread keep the plug pushed in place and slowly angle it to regulate the flow and screw it in again handtight when your draintray gets about half full then unscrew the cap and empty the tray. Give your arms a rest then repeat the process and as the flow subsides you can remove the plug completely and thoroughly clean it while the oil drips for half an hour then replace the plug handtight, empty the draintray and torque the plug to Alfa's spec.
The choice of oil is up to you Tutela 75W/90 is Alfa's recommendation, my preference is Miller CRX 75W/90 and 'zulu ferret' has suggested Redline. It's worth splashing the cash whatever you decide!
Warm the oil in a bowl of hot water then squirt far into the level plug hole until it starts to run out (should take about 2 litres) and pop back the plug handtight before finally torquing up.
Axle stand out and wheel refitted and torqued, drop the jack and remove the bricks, test run to the recycling centre to deposit the oily stuff, and JOB DONE!
Most of this is from memory, it's as accurate as I can get without getting frozen and soaked to the skin so it's E & OE I'm afraid!