This has been contributed by Big5er (Phil):

These instructions are only good for ADDING additional remotes. You MUST have one original remote for this to work. 

Step 1

Sit in the drivers seat with the door closed. You need to be able to see the side marker light/turn signal mounted on the side of the cab. (a second person would be handy or do like I did and lean way out).

Step 2

Cycle the key from OFF to ON 5 times, stopping in the ON position. The side marker light should stay on. 

Step 3

Press the lock button on your existing remote (one already programmed for your truck). The side marker light should blink, indicating you are in program mode. 

Press the lock button for the remote you wish to program to your truck. The side marker light should blink, indicating that remote was programmed.

Repeat the process for each new remote.

Step 4

Turn off the key and remove it from the ignition. Open and close your door. Test the new remotes.