To wash a Christmas sweater with lights, you should first remove the batteries from the lights and make sure they are unplugged. Then, turn the sweater inside out and place it in a mesh laundry bag. Use a gentle detergent and wash it on a cold cycle with a delicate setting. After washing, hang the sweater to dry, away from direct sunlight. If the lights are still attached, you can dry them separately using a hair dryer on the lowest setting. Be sure to keep the lights away from the heat and do not use fabric softener, as it may damage the lights.