Run a little water onto part of a clean, soft cloth. Wring the cloth out if necessary. It should be damp but not too wet.
Wipe the disk clean using the damp cloth. Start in the center in the disk and wipe in straight strokes out towards the edge. Never wipe in a circular motion.
Dry the disk with a dry corner of the cloth. Use the same straight strokes as you did to wash the disk.