As a general rule, avoid using soap and water when cleaning your cast iron cookware. Not only will it break down the pan's seasoning, but it could also increase the chances of rusting on the metal.
Try to clean your cast iron as soon as you're done using it, so the food won't have a chance to settle and harden. Start with a good sized spoonful of coconut oil and salt, then start scrubbing away.
The salt and coconut oil combine to form a soft but abrasive paste. Remember, the goal is to remove the food without scraping off the pan's seasoning, so try not to go overboard with heavy scrubbing.
When the food has been loosened off the pan's surface, use some paper towels to gently wipe away the oil paste. Depending on how much oil you used or how dirty the pan is, this may take a few passes.
After removing all the salt/oil paste, pour a small amount of olive oil in the bottom of the pan, enough to coat the inside surface. Then use a paper towel to evenly spread the oil around the surface.
Optional Step - After oiling your cast iron, put it in the oven for at least an hour at 400-500° F. Doing this helps to cook in the fresh oil and protect areas where the seasoning may have worn off.
When finished oiling (and cooled, if you chose to heat them), store your cast iron cookwares away from dust and moisture, which could lead to rusting.
Cast iron skillets, grills and dutch ovens are built to last and get better with time, use and care. Keep yours in good condition and it will provide a great cooking experience for generations.