Basically if you look at the A/C box from the driver's footwell, you'll see a black rubber tube running from the box to the firewall. That's really all there is to it.
Pull the hose off the box and gently use a blunt probe to clear out the opening in the box. Probably a bunch of water will come out. You can also use compressed air to blow down the hose to clear out the drain under the car.
Be advised that if it's blocked up, a likely cause is that you've gotten leaves and crap in your heater box. This is a known issue with Milanos that has caused interior fires for people. The fix is to remove the fan and completely clean out the heater box, and then to install a grating over the ventilation system air intake under the cowl.
Details here:
http://www.alfabb.com/bb/forums/mila...box-today.html