All moden cast iron comes with a coating that stops rust - its like a thin layer of candle wax.
The wax needs to be removed with a scourer in hot water and washing up liquid before seasoning can take place. (the metal scourers are the best) You will end up with a black/gray metal like this.
The water and moisture needs drying out. Put it on the hob for 15 minutes on a medium heat - you will see the pan sweat out all the water droplets that will evaporate.
Choose your oil. Vegtable oil or animal oil. Olive oil, flaxseed oil, lard or numerous other edible oils can be used.
Once cool pour some oil all over the pan. It needs a thin layer, get it into every nook and cranny.
With a paper towel wipe off all the oil you just put on until you can not get anymore off. There will be a fine layer left. Too much oil will smoke and set the fire alarms off.
Place the pan in the oven (top shelf if possible) on gas mark 9 or on the hob on full heat. It needs to reach over 200 degrees celsius for 1 hour. Then turn off the heat and allow to cool for 2 hours.
Your pan will have darkened brown or black instead of grey. Repeat steps 5 - 7 several times until your pan turns to a nice black colour.
With continued use the pan will gain a shiny gloss. Never put cast iron into the dish washer. Water, a little soap and a light scouring will be enough to clean it.