Crisco shortening
Temperature: 350
Crisco shortening is very highly refined, and perfect for seasoning as it won't leave a sticky residue like vegetable oil will.
Drop a small spoonful of it into the cast iron pan then place it in the preheated oven until the shortening melts. Extract said vessel, and use a paper towel or brush to smear the fat all over the pan, handle and everything.
Return it to the oven for an hour — do not drop it at any point during this process! Turn the oven off, let the pan cool down, wipe off the excess oil, and put it away. For new pans, you should do this process several time before its initial use.
Never use sharp or metal utensils when cooking in cast iron. Clean with hot water and gentle scrubbing, but avoid dish detergent, and never put it in the dishwasher or allow to soak in the sink (and if you do, just re-season from scratch). Scrub stubborn areas with salt. Dish detergent can be used, sparingly, for special needs.