When you season a skillet, what you’re doing is cooking layers of oil onto the metal. I’ve asked metallurgists what makes a skillet seasoned, and the only reasonable response I got was from a professor in Missouri who says that heating makes molecules expand, allowing oil and iron molecules to bond (a more precise term would be polymerize). The key is a clean, even heat, so don’t listen to those macho types who claim that the best way to season a skillet is on a flaming grill; that will guarantee a black coating, but the oil doesn’t bond to the iron properly and you’ll end up with black flecks of carbon on or in whatever you cook.
To season a new skillet properly, scrub it with soap and water, rinse thoroughly, dry well, rub with salt and brush with vegetable oil or lard. Do NOT use olive oil. Pop it into a medium oven, like 250, and brush lightly with more oil every now and then, say every quarter/half hour or so. Do not let the oil pool up on any surface or you’ll end up with a mini tar pit. Eventually the oil will cook to the metal. The longer you heat it, the better; the skillet might have a brownish/purplish cast, at first, but with use and proper care it will blacken. Wipe the skillet clean of excess oil, and dry it well with paper towels or do like your Granny did and use a wadded-up brown paper bag, either way using salt to scrub away the stickiness, since the surface of a newly-seasoned skillet can be tacky at first. Rub it down with salt and stick it in a paper sack for a day or two..
Don’t use the skillet for anything but baking or frying and simply wipe it with a rough cloth after use, since washing with soap and water will corrode the coating. If you scorch and have to scrub, use a non-metallic pad with coarse salt, then rub lightly with oil, and wipe again before storing.. If you’ve really screwed up, you’ll have to use a metal scrub to remove the carbon and re-season.