The easiest way is probably close the valves, then loosen one from the radiator side of the valve, put a plastic bin liner under the rad to catch the water,open the bleed valve at the top , and drain until empty, then disconnect other side from rad,and lift the rad clear, or alternativly you could drain the whole system, then close the valves on the rad you want, to remove, and re pressurise the system leaving the rad you want to work on empty,