1. Scrub off any rust with vinegar and a brush.
2. Put your pan in the oven at medium heat (450°F) until it turns back to dark blue or black.
3. Remove your pan from the oven and evenly distribute oil on the interior. This will help build an oxide layer, which will strengthen the nonstick and helps prevent further oxidation (aka rust).
Tip: Use a high smoke point oil like canola oil or vegetable oil
4. Heat the oiled pan again at medium heat (450°F) until the oil starts to smoke.
5. Wipe off any excess oil and you can start cooking.
P.S. This can also be performed on the exterior of the pan as well, but for cooking purposes, of course, the interior is more essential. If you coat the whole pan, put a cookie sheet underneath to catch any excess oil.