On the market, there is the special cleaning agent which is especially used to clean the fiber things. You could go to the supermarket to have a look. Then just add some cleaning agent into the water to clean it. The way is so easy. You could follow this advice.
The best way is not to wash it and just keep it always clean. After being used each time, do not forget to put it back into glasses box. But if it is too dirty then wash it with soft agent. Sometimes, there might be oil on eyeglasses so soft cleanse is a good choice. After cleaning, put it under sunshine and until it is dry enough.