Most folks get their cast iron cooking appliances from well meaning friends or relatives, or actively buy them for themselves. That having been done, they think that caring for cast iron--after all, it's so tough--is the same as caring for any other pan in their cooking arsenal. It ain't!
The well seasoned cast iron utensil can be thought of as the 'grandfather' to today's modern "non-stick" cookware. In many ways they are very alike. In others, completely different. Well seasoned, the cast iron pan is as "non-stick" as any of those new pans covered with artificial coatings. But in order to get/keep them that way, some different care and utilization methods will need to be [re]-learned. Before I delve into HOW it's done, let me go through WHAT to do to well functioning cast iron utensils.
Clean your cast iron utensils immediately after using them, preferably while they are still hot--although this can be done after the meal has been consumed...as they'll stay quite warm for some time. Empty them, and rinse it with hot water. Using your fingers or a spoon, gently scrape off any build-up. You should NOT have to use a scouring pad or soap. But, if you do, use one of those "gentle" pads suitable for Teflon® and a plain, "un-flavored" soap. Be sure to rinse well, as the soap "taste" will almost certainly get into the coating and can continue to contaminate the flavors of your food for some time to come.
Avoid storing food in your cast iron utensils as various food products (primarily acids & salts) can cause a breakdown of the seasoning resulting in both damaged seasoning and the food taking on an unpleasant metallic flavor.
If you have a good, seasoned, well cared for utensil, you've done well and don't have to read on. If your pan is clean & new, you should jump to Seasoning. If the instructions above in "Cleaning & Using" aren't enough, then you'll have to go through the Re-seasoning step below.
If they have them, you should also make a point of storing your cast iron cookware with their lids off (or propped ajar). This is especially true up here in the PNW where it is humid. If you leave them covered, it is possible for moisture to build up and cause rust. Should that be the case, you will have to strip the pan and re-season it.
If you get to here, it's probably because your utensil was not seasoned properly or a portion of the seasoning has worn off. If food sticks to the surface or there is rust, then it must be cleaned and re-seasoned.
Heat your pan on the stove and remove any food or other residue by cleaning the pan thoroughly with hot water and an abrasive scouring pad. If that's not enough to remove all residue and leave the surface "silver-looking" (bare metal) and clean, it may be necessary to use a wire-brush and/or sander with an 80-grit pad. Once it's been taken down to bare metal, you should wash it with hot water and a (very) little bit of soap (like "Simple Green®"). Dry immediately while still warm, and continue below.
These instructions are given for using the oven--the best method. While you can do this--with a great deal of care and attention--on a stove top, in order to get the best results you should do this in an oven.
In these instructions we shall be using lard. Other fats and oils are possible and I will be exploring them later. But for now, we're going to use the same methods that have been successfully used for centuries.
Heat the oven to ~300F (just under the smoke point for lard). Coat the utensil with lard, and place in the oven. In about 15-30 minutes, remove the pan--use care, as it will be HOT--and pour out any excess liquid. Put the utensil back in the oven, upside-down, and bake it for at least 2-hours. Use aluminum foil and/or a "catch-pan" under the utensil in order to prevent drips.
If you're going to put the utensil into service with foods high in fats, such as bacon or foods cooked with other fats & oils, then you're ready to use your utensil. If not, then I recommend repeating this process 2 to 3-times in order to form a tougher, thicker, stronger seasoned surface.