Simply a mild warm soapy bucket of water combined with a soft slightly moistened cotton towel is best. Rinse off as much dirt as possible before wiping them with the cloth as the dirt will scratch the plastic. Remember to never use paper towels on your plastic windows. Paper towels are made from wood products and they leave tiny scratches in the plastic every time you use them. Before long, you've got a fogged/hazy plastic window resulting from the use of those paper towels. No squeegees either as they'll simply drag any any dirt present across and scratch them as well.