Got this from another site. It matches the shop manual.
2003-04 Accord LX and EX with factory-installed keyless entry system
Programming the Transmitter
^ The system accepts up to three transmitters. The transmitter codes are stored in a stacking-type memory. If a fourth transmitter code is programmed, the code for the first transmitter is erased.
^ Entering the programming mode cancels all learned transmitter codes, so none of the previously programmed transmitters will work. You must reprogram all of the transmitters once you are in the programming mode.
^ You must complete steps 1-6 within 4 seconds of the previous step to keep the system from exiting the programming mode.
1. Turn the ignition switch to ON (II).
2. Press the "Lock" or "Unlock" button on one of the transmitters. (An unprogrammed transmitter can be used for this step.)
3. Turn the ignition switch to LOCK (0).
4. Repeat steps 1 2, and 3 two more times using the same transmitter used in step 2.
5. Turn the ignition switch to ON (II).
6. Press the "Lock" or "Unlock" button on the same transmitter. Make sure the power door locks cycle to confirm that the system is in programming mode.
7. Within 10 seconds, press the "Lock" or "Unlock" button on each transmitter. Make sure the power door locks cycle after you press each transmitter button to confirm that the system accepted the transmitter's code.
8. Turn the ignition switch to LOCK (0) to exit the programming mode.