There is so much wrong information in this thread. It's insane.
Just scrape all the buildup off the pan with a plastic scraper. Lodge makes one. Do this under hot water. Scrape it as clean as possible and rinse it. Your goal is not to remove the patina. Think of it as scraping everything smooth. Feel the surface with your fingers. If it feels smooth you are good. There may still be an oily residue. Rince off as much as you can; the rest is fine. Never use soap. Remember, oil is good for the pan. If for some reason you need to remove oil just wipe it off. Anyways, put it back on the stove and heat it over med/low heat until dry. This will not cause rust. This dries it quickly and prevents rust. Now that your pan is hot and dry, add a tsp of oil and rub a thin layer all over with a paper towel. Paper towel bits will not hurt you or the pan. The oil will add to the season and prevent rusting. Use only enough oil to make the pan shiny.
I've done this for years and it works fantastic.
Link to the scrapers I'm talking about but anything similar will work - https://www.lodgecastiron.com/products/pan-scrapers
Hope this helps. I love cast iron. Good cookin!