Seasoning a cast iron skillet means baking a thin layer of oil onto the metal so it hardens into a smooth, naturally non-stick, rust-resistant coating. You clean the pan, rub on a very thin layer of oil, and bake it upside down in a hot oven, repeating to build up the layers.
Wash the skillet with warm water, dry it completely, then rub a very thin layer of a neutral cooking oil over the whole pan, inside and out, wiping off the excess so no oil pools. Bake it upside down in a hot oven for about an hour, let it cool, and repeat a few times to build up the coating.
When oil is heated on iron past its smoke point it polymerizes, changing from a liquid into a hard, slick layer bonded to the metal. Each round of baking adds another thin layer, and cooking with fat over time keeps building it, which is what gives a well-seasoned pan its non-stick surface.
Use the pan regularly, dry it right after washing, and rub in a little oil before storing. Avoid soaking it, running it through the dishwasher, or cooking very acidic foods for long periods, since those strip the seasoning.
A little mild dish soap and warm water is fine for cleaning; the idea that soap ruins seasoning comes from old lye-based soaps. What actually harms it is soaking the pan or leaving it wet, which causes rust.
A neutral oil with a high smoke point works well, such as vegetable, canola, or grapeseed oil. The key is a very thin layer, because too much oil bakes on sticky.
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