If I must use restroom on plane, how do I clean seat?

Not worrisome. The airline fares limit the socioeconomic background of your fellow flyers which limits the risk of catching anything from a toilet seat (rare anyway). From an esthetic standpoint, you can use toweling supplied in the lavatory to wet and wipe and then dry and wipe. If this is a serious issue for you, you should carry a small supply of wet wipes or alcohol wipes with you to do this task.