Step 1: Remove any cooked on residue. Use a plastic scrubber, walnut scrubber, or other non-metallic option (metal can remove the seasoning). TIP: After serving food, run a bit of water in the pan to sit on the stove until you're ready to clean it.
TIP: Use coarse salt, rubbed all over with a sponge or dish cloth if you have really stuck-on food, like after browning meat.
Step 2: Wash the pan with hot water only and no soap, preferably. You can use a bit of soap every now and then, but detergent is harsh and may remove the seasoning.
Step 3: Dry the pan thoroughly on the stove. Heat it for just a minute or so on medium-high heat - not high and don't walk away! TIP: set a timer.
Step 4: Season: Remove the pan from the burner and turn it off. Using a rag (or paper towel) grab a smear of bacon grease and rub it all over the inside of the warm pan. Occasionally add it to the outside of the pan, whenever it seems to be looking 'dry' and like it needs it.
Step 5: Let cool: Set the seasoned pan back on the still warm burner and let the pan cool there before putting away.
To Maintain Cast Iron
Do these same cleaning steps every time you use your skillet and you will keep a nice gleam on the pan that tells you it's seasoned enough to meet all your non-stick needs.
Regularly cook things in it with more fat and avoid long simmering acid foods like tomatoes and lemons.
If it seems like the seasoning is wearing off, simply go through the steps to re-season the pan again.
To Re-Season (and remove rust, if needed):
Gently scrub off any rust with a steel wool pad (plain, not the kind with cleaners in it).
Wash and thoroughly dry the skillet - I set it on the stove for a few minutes on medium heat.
Coat the inside and outside of the pan with bacon grease, coconut oil, or lard.
Place the pan upside down in a 350 degree oven for one hour and then let cool in the oven.