the easiest way to clean one,is to build a small fire and turn the skillet upside down,and let it stay in the fire for a little bit. especially if the cooking surface is rough and only use wood that you would eat off in a smoker,like hickory,oak, apple,don't use any soft wood like pine,cedar and such,they will leave a resin behind
I never use any soap,the cast Iron will absorb the soap and may leave a bad taste to the food,when a Iron skillet is consider seasoned,it is the fat that has been absorbed into it and stuff doesn't stick then,I've seen it said not to use oil in them,as to why I don't know,use shortening or some lard when seasoning them
here is a link to Lodge,they make cast Iron skillets
https://www.lodgemfg.com/use-and-care/seasoned-cast-iron-use-and-care.asp
here is another link to cleaning one and it has a way to remove rust,I have one that I need to do like this to remove the rust
http://www.wikihow.com/Season-a-Cast-Iron-Skillet
they did say to use a stiff brush and soap,but that is just before you season it,hmmm now I have to find something to place the iron skillet in to remove the rust lol
ok I've got a skillet soaking in half white vinegar and water,the first picture is after i scrubbed it with a brush and soap,I have to soak it for 3 hours,then season it,I will let you know how it works out