I use my cast iron skillets all the time and although I should season it regularly, I don't and am guessing you don't either. Every once in a while we should. Charring and a little rusting is ok but when it starts to get too bad and too dry, you need to season it.
It's extremely easy just time consuming. So quickly do it, pop it in the oven and go shopping or clean the house.
I know there are dozens of ways to do this, but this is how I do it. It's probably wrong according to some of you :) but it works well for my skillets.
Preheat the oven to 250 degrees (I've heard anywhere from 200 to 350 degrees.)
Take a wadded paper towel and thickly coat the inside of the skillet with left over grease or oil. I used olive oil this morning.
It should be thick but not dripping. Place a cookie sheet or even foil on the bottom rack of the oven to catch any drippings & prevent smoking. Put the skillet face down on the middle rack and close the door. Check every 2 hours or so to see when the oil or grease is "baked" in. The lower the heat, the longer it will take but I believe better for your skillet.