Step 1: Clean and Dry Your Cookware
Normally I wouldn't use soap on cast iron (gross right?!) but would instead clean it with hot water and a coarse steel scrubber. Before we start seasoning though, we have to be sure to remove whatever the ages may have deposited on the cookware.
A steel baking pan requires less time in the oven, but must also be cleaned of any rust or stuck on stuff. Scrub the pan in soapy hot water with rough steel wool or (for really tough rust) sand it with wet/dry sandpaper.
Whether washing or sanding, the surface should feel smooth as cloth when clean. If the surface is severely pitted then sanding should help.
On cast iron I use a coarse steel scrubber under hot running water.
It's not good to let cast iron soak in dishwater as the soap and perfume oils can mingle in the porous surface and make your food taste all soapy. Lemon juice and tomatoes are acidic foods, and will strip away your non stick surface.
You don't have to remove every bit of black carbon, but if your pan has a bad soapy flavor, you probably should. The perfume oils from dish soap can soak in to the carbon layer like nasty water into a sponge.
To dry I use a towel and then I put into a 400 degree oven for maybe 10 minutes while it preheats. The pans should be REALLY DRY before the next step.
NEVER PUT HOT CAST IRON INTO WATER! IT CAN CRACK!
A note on hygiene and cast iron cookery: Always rinse your pan out before use, scrub if necessary. Don't use soap. Heat your pan to a high temperature before you start cooking, and then back it down to the desired heat. It doesn't take more than a minute, and will be as clean as you ever wanted.
Step 2: Oiling Up
Pull your pans out and unless you're using solid oil (butter, crisco, lard), you should let them cool until you can touch them without getting burned.
If you use the solid stuff, you'll need an oven mitt you can wash or else don't care about. Get your mitt on and go to town! Make sure the pan is fairly hot (like just done drying in a 400 degree oven right?).
Get some oil on to a paper towel and rub it into the metal. You will get some on your hands if you do it right. Don't leave puddles! if you can shake a drip off then there's too much. It should be a good amount, just short of too much. Make sure your racks are spaced tall enough, and put those oily pans into the oven.
Step 3: Baked on Caked On
Now with your pans happily baking in a 400degree oven, it's time to turn up the heat... To 550 or broil.
Open the windows. Turn on the fans. Keep an eye on it, and be ready to unplug the smoke detector because that burning oil is going to make quite a bit of smoke. I suggest staying in the same room to be sure everything is ok fire wise. This is a good time to put on some shorts and catch up on some dishes.
I'm really serious about the windows and fans, even in winter.
Let the cast iron stay in the piping hot oven for 20 minutes and then turn off the heat. Don't open the oven after turning the heat up! The oil needs to burn in an absence of oxygen as much as possible. In about ten minutes after turning the heat off you can put on some mitts and pull out the pans.
Leave baking pans or cookie sheets in 15 minutes or so, and then pull them out and let them cool for 2 min, then recoat and return to the oven. You can put on almost 4 layers in an hour, and give it a final hardening bake of 20 min before turning the oven off. 4 or 5 layers should be enough to give your bakeware a long lasting non-stick surface.