Scrub a new unseasoned skillet with steel wool or a wire brush to removed the protective wax coating then wash with mild, soapy water. Avoid using soapy water on the pan after this.
A preseasoned pan should also be seasoned before use. Do not wash with soapy water, instead use a stiff brush and scrub the inside under very hot water. Then dry the pan on the stovetop.
Use a paper towel to rub the skillet with lard, Crisco, or bacon fat. It is best not to use a liquid oil like vegetable oil because it leaves a gummy residue and goes rancid faster. Do not use olive oil or butter because they will smoke and burn very quickly.
Put the greased skillet upside down in a 350ºF oven (this way the fat doesn't pool at the bottom) for about an hour. Put a sheet of aluminum foil on a rack below the skillet so excess fat can drip onto the foil.
Let cool before use.
Note:
Avoid cooking wet or acidic foods, like tomatoes, in a cast iron because it will slowly dissolve the seasoning.
After cooking, do not wash cast iron with soapy water. The soap will ruin the seasoning and the taste will absorb into the porous iron and impart a soapy taste to food.
If there is food stuck to the pan, pour some kosher salt into the pan and use it to scrub away the food particles.
Always dry the pan throroughly before storing because water will cause the iron to rust. The most effective way to dry the pan is heat it on the stove top. Paper towels and dish towels may be used but it might leave fibers on the pan.
If the pan begins to rust, use steel wool or a wire brush to scrub it off and reseason.
I love to cook bacon in my cast iron because as the bacon cooks, the pan gets seasoned at the same time.