How do i Program a ford ranger key fob?

Answer

These instructions work for a 2003 Ford Ranger and several other years with the PATS^* key.

Sit in the vehicle with the doors closed with a PATS key that's already programmed for the vehicle and ALL key fob remotes you want to work with the vehicle including new un-programmed key fob remote.
With the doors unlocked, insert the key into the ignition and turn it to the on position eight times in quick succession. When doing this, do Not turn the key far enough to start the engine and don't turn it all the way to the steering column lock position.

........1.............2.............3.............4............5.............6.............7.............8.
off...ON...off...ON...off...ON...off...ON...off...ON...off...ON...off...ON...off...ON

Leave the key in the "ON" position on the eighth time.
The door locks will automatically LOCK and UNLOCK indicating the vehicle is now in the Key Remote programming mode.

Finally press the LOCK button on each remote (Key Fob), one at a time.
When you do this, the vehicle will respond with a quick down-up of the door lock buttons for each remote. Remove the key from the ignition and try the remote(s) again to make sure the programming was successful.

Make sure you have ALL remotes that are going to be used with the vehicle with you when you follow the above instructions. It isn't actually the key fob remote that gets programmed. Its the vehicle's computer that is programmed to accept the unique number that is hard coded into every remote. A programming session doesn't ADD remotes to the computers memory, it WIPES OUT the old remote memory and replaces it with just the remotes programmed in this session.

Note: The above instructions replace instructions here that were for programming a new PATS key. The author of those instructions claimed not to know what a "fob" was. In case someone reading this actually wants to program a new key, here are the instructions:

If you need to program a new PATS key, you must have TWO keys that already work. If you only have one, you can not program your own second key and must go to a Ford Dealer.
Insert one working key and turn the vehicle on and off. Insert the second key and turn the vehicle on and off. Insert the new PATS key and turn it on and off. It should now be programmed.

^*PATS is an anti-theft system Ford uses that uses keys with a built-in computer chip.