Never had an automatic Alfa but the fundamentals of the DeDion setup should hold true for the manual setup.
If the new flex coupling is on the driveshaft, does it still have its metal band? If so leave it on until after installation. If not, screw two hose clamps together and make a new one, tighten the clamps firmly but do not wrench down in excess.
Unless already removed, clear away the exhaust, shift linkage, and center cross brace.
My Milano has no pinch bolt so I defer to Kredden's advice on that. I've never had to jack the engine but that does not mean it won't help. I defer to Kredden again.
Connect the driveshaft to the engine before you remove the six bolts that support the transmission to the frame at the front of the DeDion axle and lower it as described by Kredden and me below; make sure you get the center support bearing over the two mount studs.
Then remove those six bolts Kredden references that attach the the transmission to the frame at the front of the DeDion axle and using a floor jack, raise the rear of the DeDion tube from the back of the car, thus lowering the front of the axle.
Then offer the rear flex coupling to the spider yoke attached to the transmission. They will not mate flush, rather only at the tops at this point; not to worry. Slowly lower the jack at the rear of the car that lifted the tube, this will have the effect of flush-mating the rear flex coupling to the transmission and pushing the driveshaft forward and into position up front where you can install the attachment nuts.
From that point it's a matter of rotating the driveshaft using a wrench and installing the six bolts (three at each flex coupling) that mate the driveshaft to the engine and transmission. Do not tighten these self locking nuts completely just yet; snug them lightly at each end, then only when all are flush and gently snug at each end, tighten them fully.
Do not tighten the two center support bearing nuts until both coupling ends are fully tightened in this manner. This allows the driveshaft to center as best it can with the least stress on the flex couplings. I've never run the car with the attaching nuts loose so I don't know if that helps or not; as I stated earlier I also have no pinch bolt. I can only say that in my car's configuration (as well as my Alfetta) my technique has not resulted in any vibration or premature coupling failure.
Reinstall the shifter linkage, center cross brace, and exhaust, then take her for a drive!