It's use is fairly simple, I just spread a thick layer of boiled linseed oil on the metal with my fingers and then let it dry. Depending on how thick you apply it, it can take days or weeks to fully dry.
Back when I was blacksmithing, sometimes we would throw the piece back on top of the fire to "season" the oil black (similar to seasoning a cast iron frying pan).
Another method we used to finish metal was something we called the "Stink Rag" method. This can be dangerous if the metal is too hot. We would heat the metal in the forge (hot, not red or colored hot), then rub it with an old shop rag that was infused with boiled linseed oil. (wear thick gloves) If the metal is too hot the rag will flash into flames in your hand (BTDT). This needs to be done in a well ventilated area, it produces A LOT of smoke and fumes. Thus the stink rag reference. It works best on small pieces, large pieces are difficult to heat properly.
You may be able to duplicate this finish by rubbing the metal with a thick layer of linseed oil, then burning it with a "clean" flame on a torch. Once everything cools down, lightly rub the metal clean with a rag and then apply a thin coat of linseed oil to seal it all. (never tried it myself, just a theory)
