Coat with a light coating of shortening or lard. Oil or butter does not work well. Put in a moderate temperature oven (350-375) for an hour or two...everyone does this part different.  My grandma puts it in a low temp oven (200-250 for about 10 hours...). I let it cool in the oven when done usually, but not always. I like to repeat this process a couple times at least if possible...it really is not a touchy thing...you will lose your seasoning if you wash your pans in soapy water. Sorry to say so to those who think it is "dirty" to not use soap on them but for folks who can't stand not to have soap on the pan cast iron is not a good idea. It is best to if possible just wipe the pan with a towel...if you just used it for bread or whatever. If I have to I will use hot water, and only use soap if I MUST. Sometimes soap affects the seasoning, sometimes it doesn't. I think it depends on how long its in the soapy water and how much soap and how much you rub/scrub the pan.

 

When you are done cleaning it, dry it immediately in a warm oven or on a warm burner. It is best then to wipe it down with some shortening or other fat...

 

If you cook tomato or other acid based foods in your pan you MUST re-season it...the acid will strip off the seasoning...

 

If your pan is not seasoned well you are much more likely to get rust on it...and have that mess all over again...

 

I have a whole drawer full (stove drawer) of well loved cast iron pans, skillets, dutch oven, etc...I love these pans and use them a LOT...)