How do I reprogramme my key fob?

I have a Ford focus ghia 2000 plate. I have recently replaced the battery in the key fob. The central locking working erratically at first and now will not work at all. The key will however, work manually. How do I reprogramme my key fob.
I have tried the following:
Fasten seat belts, Turn ignition from 1 - 2 4 times in 6secs, switched the ignition off. Then held down one of the buttons but no acoustic signal was heard and fob still does not work