You can tell that the pan needs to be re-seasoned if after washing, it no longer retains its black glossy appearance. It will look dull and grey. This happens when the pan has been used to cook something with a lot of liquid, such as spaghetti sauce.
Step one -- wash the pan and dry it immediately with a paper towel (do not use cloth because it stains).
Step two -- place it on a burner on the stove and put the heat on high.
If there is any water still in the pan you will see it evaporate. (Never let the pan sit with water in it -- this will cause rust stains -- but you can use this method to remove, those also.)
Step three -- add a tablespoon of your favorite cooking oil or fat and let it melt -- making sure to spread it over the bottom and sides of the pan with a spatula as it continues to heat up
Step four -- step back and watch the inside bottom of the pan. When the cooking oil or fat starts to take on the shape of the burner beneath the pan, keep a sharp eye out because it is about to begin smoking.
Step five -- At the instant it starts to smoke turn the heat off (remove from burner if electric) and set it aside to cool down completely.
Do not use the pan until it has had the chance to cool down to room temperature or it will not be non-stick. It becomes non-stick when it has cooled and absorbed the cooking oil into the surface. After that, you can again cook without things sticking to the bottom of the pan.