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 said that heat makes molecules expand and allows oil and iron molecules to bond, or 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 quarter/half hour. 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. Use salt to scrub away the stickiness, since the surface of a newly-seasoned skillet can be tacky at first. After the salt scrub, place the skillet or whatever cast iron utensil you’re seasoning in a paper sack for a day or two.
Wipe cast iron with a rough cloth after use, and be sparing with soap and water washings. Always dry thoroughly. 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.