So you got your hands on an old cast iron skillet. An epic barn find, an inherited piece from a family member or friend, maybe even a flea market bargain, and its condition is less than perfect... I'll help you get that skillet in tip top shape in no time. The "best" way to do this depends on who you ask, but this is what I do and I find it to be a very quick method.
If you are starting with a very rusty skillet, I like to give it a quick rinse and scrub just to remove any loose debris and place it in a 400° oven for about 20 mins and shut the oven off to let the skillet cool slowly until you can handle it with bare hands.
Now, get your tools ready to go. Initially, you're only going to need a brass scrub pad, water, and a ton of elbow grease. Scrub all old chunks off, paying particular attention to the outside too. Don't stop until all surfaces are clean, smooth, and don't worry and any minor pits from rust. Major pitting cannot be repaired easily and if your cast iron has major pitting, you now own a decoration, I wouldn't use it to cook with.
Back into the oven, at 350 for 20 mins, and then apply a small amount of bacon grease. You can use other oils like coconut oil, olive oil, shortening, etc. but part of the allure of cast iron cooking is the flavors you can achieve, and bacon makes everything better.
Rub the oil/grease all over the skillet with a paper towel, making sure to apply a coat to the entire surface, including the handle. Back into the oven for 20 more minutes. Remove it and rub another thin layer of oil to the whole thing. Back in the oven, repeating this step 2 or 3 times, 20 mins each. On the last treatment, just turn the oven off, letting the skillet cool slowly until you can handle it. Wipe with a clean paper towel to romove any excess oil/grease. You are now ready to use your iron to create tasty vittles.
After each use, I clean the skillet by simply scrubbing with water and a nylon brush. Usually this is all that's needed. If you have any carbon buildup that doesn't come off with gentle scrubbing, no worries...just heat the iron up again, adding a bit of grease, and a liberal amount of COARSE sea salt. Using a paper towel, scrub then pan until all then bits are gone. Rinse to remove the salt.
Before storing the skillet apply a thin layer of grease again to keep the pan from rusting. I use my skillet so often (it's actually my favorite pan!) that I don't ever really have to worry about the pan rusting.
When cooking with cast iron, proper heating and oil choice is key to the success of the finished product and achieving/maintaining the nonstick properties of your pan, but that is for another post. Happy cooking!
