How to Season (or Reseason) a Pan
The interior of a new pan has a roughly textured surface, but a fully seasoned skillet is smooth and black. Every time you use the skillet, as long as you clean it properly, you'll start to bond the oils and fats from the cooking into the surface. If you have a pan with an enameled coating on the inside, it will not season. The enameling process eliminates the need for seasoning.
Seasoning the Unseasoned Pan
The first step is to remove the paraffin-like or shellac coating on the pan that will prevent your skillet from seasoning properly. The best way to do this is by washing and scrubbing it in hot soapy water. Be sure to clean every edge, the handle, and the outside of the skillet thoroughly. Dry it completely before proceeding. Place the skillet over a medium burner and let the water evaporate. If you smell a strong, chemical smell, wash and dry the skillet again.
Now that you have a clean pan, you're ready to add the first seasoning layer. Preheat the oven to 350°F and place a layer of aluminum foil or a baking sheet on the bottom rack of the oven. Cut off a tablespoon-sized chunk of shortening and rub it over the entire surface of the skillet, inside and out. Place the pan upside down in the oven. Any excess will drip off the pan as it bakes. Set your timer for one hour.
You'll likely notice a slight burning smell and see some smoke. This is normal and should not be a cause for concern. The oil you applied is smoking and the pan is soaking up the oil and the carbonization that it creates. Once the hour is up, turn off the heat and leave the pan in the oven to cool overnight. The surface should be darker, but it won't be black and shiny yet. It will take several uses and proper cleaning to get the desired appearance.
If you have a glass top range, don't drag the pans across the range. This will scratch the surface. Because it takes a while for electric ranges to cool down, move your cast-iron pan off the burner if you want it to stop cooking. Alternately, if you're close to finishing a dish, you can turn off the burner but leave the pan in place so it finishes cooking slowly.
Cleaning and Prepping the Seasoned Pan
There is some disagreement about whether pre-seasoned skillets are worth their greater cost. For the occasional cook, not having to go through the steps described above for an unseasoned skillet may make the higher price worth it. You just need to wash a pre-seasoned pan without soap using hot water and a stiff-bristled brush. Dry the pan completely, pour some vegetable oil on a paper towel, and rub it over the surface of the pan until you're ready to use it.
Cleaning and Prepping a Healthy Older Skillet
If the inside of the skillet is smooth and shiny, all you should need to do is rinse it with hot water, dry it thoroughly, and wipe it with a fresh coating of oil. If you're concerned about latent bacteria, place an inch of water in the pan over high heat. Let the water boil for one minute. Hold the pan over a sink and swirl the pan around so the hot water coats the interior. Then simply dry it and coat it with oil before using.
Prepping a Needy Older Skillet
After you've stripped the rust or corrosion off the skillet you'll need to follow the same seasoning steps as you would for a new pan. If the skillet seems dry and not glossy after the initial seasoning, it may mean the oil used in the seasoning was soaked into the surface of the skillet. It may be beneficial to run it through a stovetop seasoning step before cooking in it. Pour some vegetable oil on a paper towel and rub it over the entire surface of the skillet. Place the skillet over medium heat and let it cook until the oil starts to smoke. Turn off the heat and let the skillet cool to the touch before using.