Start by disconnecting the negative cable from the battery, remove the serpentine belt from the alternator, disconnect all the mounting bolts, and disconnect all the wires from the alternator, remove alternator.
It really depends on the car. If it has a newer transverses engine more than likely it will be a lot more difficult to change it. Especially if it has the alternator mounted down low in the engine bay. Older normal mounted engine found in rear wheel drive vehicles are typically much easier.
