It is in your owners manual. I beleive it goes something like turning the igniton to on(but not starting the truck) 8 times in like 10 seconds, and the locks should cycle, then you push any button on the remote, and they should cycle again to show that that keyfob is programmed.
I am not sure if this is exact, but its close. Also you will have to do all remotes at the same time. Even if you already have one programmed, whenever you add a remote, it erases all previous ones so you need all of the at the same time.
2008 Explorer XLT