According to the Captain, the air carrier flight was descending out of flight level 240, when he asked the flight attendants to secure the cabin and take their seats because of anticipated turbulence. Two to three minutes later, the airplane was at flight level 200 when it experienced moderate to severe turbulence for several seconds. According to the flight attendants, they were standing and completing the prelanding requirements at the time, when they were unable to maintain their balance. One of the flight attendants sustained a serious foot injury due to a fall, while the others were not injured. The airplane was not damaged and landed uneventfully at the destination airport.