My Grandma’s 90 year old cast iron looks like that and I have no idea how to season it.

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.

Rapeseed the pc term is struggle snuggle seed oil

Canola oil.

Canola is a modified version of rapeseed developed in Canada. I might see a bottle or two of rapeseed oil next to the 4L jugs of canola from 3 different brands in stores here but that’s apparently not the kind of ratios you’ll see in other countries across the pond.