Get in car, close all doors, ensure all seatbelts not plugged in. Switch ignition to position 2, do not start engine, plug drivers belt in and out 9 times within 60 seconds, ending with belt out. Car will give a beep to confirm. Remember it disables the warning light too.
If you don't want to deactivate the chimes altogether but still want it to warn others (ie the wife) then with the engine running, plug the clip in and release it twice in succession. This turns the chimes and warning light off for that journey. It will reset when you switch off the ignition. I found this to be the case with my car anyway!!