I Wrote a Cast-Iron Cookbook—Here's the Right Way to Season a Cast-Iron Skillet
Wash the Skillet with Warm Soapy Water and Dry Well: While using soap on cast iron was frowned upon for years, it is, in fact, OK. After washing, rinse off any soap, then dry the pan thoroughly with a soft cloth. The skillet is raw iron at this point, and water will cause it to rust.