I found this:
1) Have 2 working keys, and close all doors (some models).
2) Insert key A into the ignition
3) Turn ignition to the ON position (don't start the car) and wait for some amount of time (normally 1 - 5 seconds or so).
4) Turn the ignition back to the OFF position and wait for some amount of time (again, normally 1 - 5 seconds or so).
5) Possibly repeat step 3 and 4 up to 4 more times (depending on model).
6) Remove key A and insert Key B into the ignition.
7) Repeat steps 3,4 and 5 with key B.
8) Remove key B and insert the unprogrammed Key C and turn the ignition to ON (again don't start the car yet).
9) Wait for some amount of time
10) On some models start the car with new key C, but doing this on some others will terminate the programming sequence a this key.
10) If a Key D (or E) is required , repeat steps 8, 9 and 10 with Key D (or E).