for a 99+ you turn the car from "off to run" 8 times in 10 seconds. leaving it in the run position. the locks will lock then unlock to tell you it is in programming mode. hit a button on the first transmitter you have. (if you have another hit a button on it now.) once finished turn key to "off" the locks will lock then unlock telling you your done.
if 94-98 look in the trunk under the carpet panel on the driver side. you will see a big black looking thing. on the fron there should be a little grey looking plug thing. inside is 2 metal prongs. first turn key to "run". you must "jump" these two together.) i used a fork.
once the connection is jumped the locks will lock/unlock to tell you it is on programming mode. then hit a button on transmitter. once done turn key to "off" and it will again lock/unlock to let you know you done.