Expose molten ferrous metal to … a magnet.

Welp…

Magnets are created by running an electrical current through a material, so there is no need to have a ‘first magnet’. This is happening ‘naturally’ in the earth core, in the sun, and in other stars. (https://physics.stackexchange.com/questions/565245/how-was-the-first-magnet-made)

So you need to look around and find some magic rocks.

Natural magnets, called “lodestones”, were found in iron ores (magnetite) from the ancient region of Magnesia, hence the name “Magnet”. (https://physics.stackexchange.com/questions/615500/how-did-magnets-first-come-about)

Maybe the sword with the stone was just a big lodestone with a sword sized hole in it. Just throwing that out there.

And one more cool fact…

Based on his discovery of an Olmec artifact (a shaped and grooved magnetic bar) in North America, astronomer John Carlson suggests that lodestone may have been used by the Olmec more than a thousand years prior to the Chinese discovery.[23] Carlson speculates that the Olmecs, for astrological or geomantic purposes, used similar artifacts as a directional device, or to orient their temples, the dwellings of the living, or the interments of the dead.[23] Detailed analysis of the Olmec artifact revealed that the “bar” was composed of hematite with titanium lamellae of Fe2–xTixO3 that accounted for the anomalous remanent magnetism of the artifact.[24] (https://en.wikipedia.org/wiki/Lodestone)