To season your cast iron skillet you need to first wash and scrub the cast iron skillet out very good with soapy hot water.
After washing and scrubbing out the cast iron skillet make sure to dry it out throughly and then spread a thin layer of vegetable oil or shortening over the cast iron skillet.
After doing that turn your oven on to 375 F and place the cast iron skillet on the middle oven rack and then place some tin foil on the bottom rack to catch any drips from the oil and then leave the cast iron skillet in the oven to bake for 1 hour.
Then after the hour let the cast iron skillet cool down in the oven.
This will season or reseason your cast iron skillet.