BLESS THIS MESS
Wash your cast iron in hot soapy water. If they are rusty or coarse use a copper or untreated steel wool pad to really scrub them. You can scrub inside and out.
Rinse very well and then pat dry with a dish towel or paper towels.
Evenly coat the inside and outside of the pan with vegetable oil using a paper towel. This is just a light layer, no need to put it on thick. It should be darker in color but not dripping with oil.
Preheat your oven to 325 degrees F. and place the pan upside down in the hot oven. Let the pan bake for one hour. Expect it to be pretty smoky.
After one hour, turn off the oven and let the pan stay in the warm oven until it has cooled (that will take a few hours). Your cast iron is now clean, seasoned and ready to use.