There are 1000 ways to do this but this is the best way that I have found. It works every time. I even took an old skillet that I got from my mother that was rusted and in horrible shape. Sanded off the rust, cleaned it up with soap and water and followed these steps and I am still using it to this day. The more you season your cast iron the better it will perform.
Preheat your oven to 450°F. Wash the skillet with warm water (and a tiny bit of soap if you have to) and dry completely. Put it in the oven dry, no oil of any kind and heat it up. Leave it in long enough to reach the oven temperature. Be careful and remove the piece from the oven and let it cool to where you can just handle it. Use Crisco shortening only and use a cotton rag to apply a very light coat on the entire piece. You want a very light coating to keep the oil from puddling up in the skillet. Once completely coated put it back in the oven at 400°F for 30 minutes. After 30 minutes turn off the oven and leave it in the oven till it cools on it's own. A new pan will be now be brown and shiny. An older pan should start turning darker until its black and shiny. That's when cast iron is at its best.