Remote door opener problem

My wife lost one of our remote door openers and the other one seems to have died. Put a new battery in it and still nothing. I was at a car swap meet when I saw a table full of remote door openers. The fellow was selling them as working. I thought they were somehow matched with a particular car through programming. He tells me that I can do it myself by holding down the door lock button and following the instructions in my manual. That didn't work. He seemed to think it could be done when I asked him because I was a bit pessimistic. What do I have to do to program these two to work with my car? I think the process is different from the process one would follow if they were reprogramming the original ones that came with the car.
Thanks.
Bob