Most surefire way I know is preheat your oven to 450, put a tiny but of canola, rapeseed, or another neutral oil on it, wipe off as much as you can with a paper towel and toss it in the oven for half an hour, the nrepeat 3 or 4 times. When I say remove as much oil as much as you can, I mean the towel should come away juat about dry. Then to cook with it, let it get hot first, add some form of fat, butter, oil, bacon grease, etc. and then add your food. Waiting for it to get hot first is the key.

get the grill ripping hot outside (500 degrees)

put cooking oil on a paper towel and swab the whole cast iron pan

Throw it on the grill until it stops smoking

pull it off, let it cool a little and swab it again (super thin layer)

Throw it on the grill until it stops smoking

repeat until you're happy with the color

Seconding Pissman's advice. The only other tip that I know of is to encrust it with salt as well as oil, but that seems to be expensive and unnecessary. Honestly just cooking several pounds of bacon in it will do the same thing.

Preheat the oven to ~450-500, throw the pans in for five minutes to drive out any moisture.

Pull the pans out, and apply a very thin coat of oil using a paper towel or lint-free cloth. Flax oil is best, low-temperature oils in general are better than frying oils here. Put the coated pans back in the oven for ~45-60 minutes, then remove. Repeated coats will significantly increase the strength of the coating