How do you get the alternator out of a 2003 Focus SE, 16V Zetec DOHC?

Disconnect battery ground cable
Remove the ground wire (black) from the alternator
Disconnect the positive wire (red wire held on by a retaining nut) from the back of the alternator
Now disconnect the wire harness from the alternator
Remove the drive belt
NOTE: Some models use a belt tensioner that applies tension to the drive belt.
The best tool to use for this job is the serpentine belt too.
If you can borrow one from a neighbour you can get one at PartSource. (Part# 648451)
On other models you will have to loosen the adjusting bolt on the alternator bracket.
If you're not sure about how to remove the drive belt, visit your local PartSource for free print outs of car specific instructions.
Take off the mounting bolts from the alternator bracket
Now remove the alternator from the engine compartment

Installing Your New Alternator
Put the alternator in the engine compartment
Install the mounting bolts that connect the alternator to the alternator bracket
Connect the alternator wiring harness
Now connect the positive wire to the alternator
NOTE: Your new alternator should come with a retaining nut.
Remember to remove the nut before you install the positive wire.
If it doesn't keep the old nut.
Connect the ground wire to the alternator
Now install the drive belt
This is a great time to replace your old worn out belt. When inspecting your old belt look for signs of cracking, oil soaked, hard glazed, splitting or fraying.
All these signs mean it's time you replaced your drive belt.
If your vehicle no longer has the belt routing diagram under the hood, visit your local PartSource for a free print out of the belt routing diagram.
Reconnect the ground battery cable .