Properly seasoning an old iron skillet??

I have 2 items I inherited from dearly-departed relatives : one, my grandmother's one & only heavy duty handmade rolling pin which she received as a wedding gift in 1908 from her uncle who was a millwright by trade; and, the 2nd. item is an iron skillet from about 1915 that belonged to my grandmother's sister-in-law. She died about 25 years ago and the skillet has not been used since then but is in very good condition with very little rust. HOW do I clean / season it so I can start using it???

Thanks,
LarryD