I do it about every time after I make my steakhouse steak, because some pepper corns get stuck to the bottom of the pan and to remove those, I usually let the skillet sit with some water in it. Which is not good to do, as your skillet may get rusty. Oh well, hence the seasoning - re seasoning.
Let's get to work:
Prepare your pan by scrubbing it with some soapy water and make sure there is no rust or food residue left.
Apply a coat of melted shortening on both the inside and on the outside. Do not substitute the shortening by anything, please, as it will burn and smoke.
Preheat your oven to 350 F.
Line a sheet of aluminum so you can catch any dripping.
Place your skillet upside right on the wire rack.
Bake for 20 minutes and if it smokes reduce the heat by 10 degrees.
Remove the skillet, wipe any grease excess and return to the oven, this time placing it upside down for 2 to 3 hours.
Turn the oven off when the time has passed and let the skillet cool in the oven overnight. Or in the course of at least 3 hours.