to season old cast iron use something like steel wool, etc to scrub off as much of the rust as you can, then coat them down with oil. (I use vegetable oil), then put them in an oven and turn it on to around 400 degrees. Let them heat up and once they reach temperature turn the oven off and let them cool. I will then recoat them with vegetable oil, let it soak overnight and then wipe down. For one with no seasoning left, I might do that a couple of times, then I try to fry something in them the first couple of times I use it. I then leave the oil in the pan overnight and then rinse it with just water, wipe it dry (water is the enemy here) and then rub a very thin coat of oil on the pan and wipe dry. It should be in good shape for you after that. Remember when you wash use little to no soap, rinse well, dry thoroughly and then recoat each time with a thin layer of oil. This will keep it in good shape.
The Lodge store has a great section on care of cast iron.