I love, love love to cook in cast iron. I have a small collection of skillets, deep pans, and one Dutch Oven that I found at thrift stores, Goodwill and discount stores. Here in Oklahoma the main ones are Walls and Big Lots. The new cast iron are pre-seasoned and good to go right out of the box, however, if you find a treasure at a thrift store you may have to season it before you use it. This is my method.
Pre-heat the oven to 150 F. Place the cast iron item with no oil in it in the oven for 15 minutes. Take the cast iron out of the oven and liberally coat it with oil only on the inside. I prefer to use Extra Virgin Olive Oil. Some people prefer Crisco, vegetable oil, canola oil. Use what you prefer. Return the cast iron to the oven and raise the heat to 225 F. Leave it in there for 45 minutes. Remove from the oven and wipe the excess oil out of the cast iron. Buff the inside to a dull shine and let cool on your stove top. After it is cool, wipe the inside down with a thin coating of oil and store. Cast iron likes to be used, often. If you don’t use it, the oiil can become rancid. If this happens, heat the cast iron, wipe out the rancid oil and reseason.