Wash the skillet with warm water and a stiff barush (mild soap is fine for a deep clean — it won't strip seasoning after the initial cure).
Dry completely, then place on low heat for a few minutes to evaporate any remaining moisture.
Rub a thin layer of high-smoke-point oil (vegetable, canola, or flaxseed) over the entire surface, inside and out, then wipe off the excess — the pan should look almost dry, not greasy.
Place upside down on the middle oven rack with foil below to catch drips, and bake at 450–500°F for one hour.
Turn off the oven and let the skillet cool inside before removing.
Repeat the oil-and-bake cycle 2–3 times for a new pan, and re-season anytime food starts sticking or the surface looks dull or patchy.