Into your pan, pour a quarter-sized dollop of flaxseed oil. Use your hands or a folded paper towel to wipe the oil all over the inside surface of the pan.
Now, take a clean rag or paper towel and wipe ALL of the oil off- it should look fairly dry, not glossy at all. And although it may look like there is no oil, there is a thin layer of oil on the pan. And thin is what you want! The best seasoning comes from building many thin layers. Resist the urge to put a thick layer of oil on the pan.
Place the pan upside down (no need to put a pan under it, as there should be nowhere near a thick enough layer of oil for it to drip) into a cold oven and then turn the oven on to 450 degrees F.
Cook for 75 minutes, then turn the oven off and allow the pan to cool in the oven without opening the door (should take about two hours).
Repeat: add another thin layer of oil then wipe it all off and cook again.
Before using your pan to cook in, you will need to build anywhere from 6-10 layers of seasoning in this manner. You can do this over as many days as you want, no need to build the layers all in one day.
Recipe by Whole-Fed Homestead at https://wholefedhomestead.com/the-everything-cast-iron-guide-how-to-buy-clean-use-restore-and-season-cast-iron-pans/