I expect that the "Open Fire" method (the second technique in the linked article) may be your best bet.
Create a fire pit large enough to meet your needs. Make sure you allow for enough room around where you will place your device to allow the fire to properly breath. Place (or devise) a grate to support your cast iron and build a fire. (You might want get some Coal (real coal...not charcoal...reach out to a local blacksmith if you don't know where to get it.) and go through several repetitions of the process.
1. Build a fire up until you have a bed of red embers & a low, non-sooty flame. ( 30 min. ) This kind of fire is nearly smokeless & hot as all get out.
Set the clean iron over the fire & heat ‘till it turns “white-blue” in color. This means it’s ripping hot, and you’re now ready for the first coat of oil !
Hook the handle, pull the pan off the fire, & mist it evenly with the Flax Oil.
Not Too Much! You don’t want puddles, drips or thick coats. Just a very thin & even layer into every nook, cranny, handle & backside. The pan should be smoking fiercely when you do this. Stand down wind.[sic] (added: stand UP Wind so the smoke is blowing away from you.)
Put the smokin’ pan back onto the fire & let it smoke out until it stops. Watch for uneven heating & adjust the iron over the best heat spot on your fire.
You’ll know you are ready for the next layer of oil when you see the ash of the fire wisp off the face of the once sticky oiled surface. The color should be even, the first few coats have a brownish-red hue on them, & the pan will look dry again.
Repeat steps #2-#5 about 6 times ! You’ll see the pan turn to a rich “blue-black” by the last round of oil. (Approx. 1 hour of seasoning.) Enjoy the fire when you’re all done & marvel at your crafty work while letting your iron cool down naturally on a wire rack.
Be very careful to not put the hot iron onto something wet or cold ! The dramatic difference of temperatures could cause a cast iron skillet to crack or warp from thermal shock!Your pan should be a deep black color and ready to use within one hour’s time of this open-fire process.
If you find that the cookware is still a little sticky after it’s cool, you may need to oven bake it for 30-45 minutes, to finish it off and get it totally dry.If your pan develops of reddish color and you can’t seem to get it black, there’s three possible issues you’re facing:
You probably don’t have enough heat on it
You haven’t done the seasoning long enough
You put the oil on too thick
(Copied from linked article, should the link die in the future)