1. If the pan is new, be sure any adhesive label is completely removed.
2. Wash with very hot water, rinse and dry the utensil.
3. Grease the inside surface with Crisco or other solid shortening. A medium-light coasting, as you would grease a cake pan, is sufficient.
4. Put greased pan in preheated oven 300 for 1 hour.
5. Remove, cool and store the pan.
A skillet/utensil can be seasoned as often as necessary to maintain surface. (i.e. After making tortillas, after all that heat, the surface of your skillet looks dry, just season it again before you put it away)
Refrain from using soap or detergent when cleaning. Seasoned cast iron may be cleaned very nicely with boiling water and a stiff bristled brush. A short-handled brush with stiff nylon bristles used only for cleaning cast iron skillets is a good idea. Often, say after making cornbread, dry brush the skillet vigorously, wipe it clean with a damp sponge, and dry it with a paper towel. It is important to dry cast iron utensils well after use; they will rush unless throughly dried. Cast iron utensils will darken with use, turning from a steely gray when new, to a dark gray or black.
UNTIL the pan is very well seasoned, either by many uses or repeated seasongs, do not attempt to cook foods with a high acid content (tomatoes, for instance). The corrosive nature of high-acid foods will not react well to unseasoned cast iron. Once a pan is well-seasoned, however, you can use it for just about anything.