Obtain two or three large metal bowls. The bowls should be easy to carry water in so you do not spill it when carrying them into the room with the radiator. Shallow bowls seem to work a little better in terms of creating vapor that rises into the room.
Fill your bowls with tap water.
Place one of the bowls on top or near the radiator. Place the rest of the bowls in a few corners of the room. The water from the bowls will evaporate, putting moisture in the air. Make sure to put the bowls in locations that are inconspicuous and not in the way.
Fill the bowls with clean tap water on a frequent basis.