This is the video I have used in the restore cast iron.
Restoring Cast Iron
Hope this helps! Lauren - The Little Lake Cottage
Depending on how bad it is You can start and try a half of cup of salt on a 12 inch pan or a little more spread around and a potato cut in half and scrub it all around Rinse the pan with clean water, then dry with a paper towel and place your pan on the stove over low heat so no moisture remains. then you might try white vinegar and water half of each then let it set for an hour or two maybe more scrub it away with your abrasive scrubber like them round copper ones until it is gone then rinse it and dry it how ever you do it then I always after using and cleaning it I rub or spray oil or lard around in it . and leave it there until next time always keep them coated I do.
Try this method: https://www.craftylittlegnome.com/clean-season-cast-iron-skillet/
Here are several methods for you to choose from.
Hi Myamie - This is a great tutorial on how to clean, season, and care for cast iron skillets. Hope it helps! Hugs, Holly