Unless it comes preseasoned, a carbon-steel pan requires seasoning just as a cast-iron pan does. This process bonds oil to the surface, providing protection against rust and making the pan nonstick. The instructions that come with our winning pan suggest an unusual method, but we found that it really works.
Initial Seasoning
First you’ll need to remove the new pan’s wax or grease coating (used to protect the metal from rusting in transit). Use very hot water, dish soap, and vigorous scrubbing with a bristle brush. Dry the pan and then put it on low heat to finish drying. Add 1/3 cup oil, 2/3 cup salt, and peels from two potatoes (these help to pull any remaining wax or grease from the pan surface). Cook over medium heat, occasionally moving the peels around the pan and up the sides to the rim, for 8 to 10 minutes. (The pan will turn brown.) Discard the contents, allow the pan to cool, and wipe with paper towels. You are ready to cook. (If you experience sticking, repeat once.) This method will work on any carbon-steel skillet.
Maintenance
Avoid soap and abrasive scrubbing. Simply wipe or rinse the pan clean, dry it thoroughly on a warm burner, and rub it with a light coat of oil. If you accidentally scrub off some of the patina, wipe the pan with a thin coat of oil and place it over high heat for about 10 minutes until the pan darkens (it will smoke; turn on an exhaust fan).
Blotchy is OK
As soon as you season and start cooking in a carbon-steel pan, it changes from shiny silver to brown and blotchy. The blotches are a sign that the pan is building up a slippery patina, which will help it become increasingly nonstick. The blotches and nonstick capability may initially wax and wane, but with use, the pan’s cooking surface will gradually darken and become more uniform in color.
Source: From Cook's Illustrated | September/October 2015