Lord Dunsany explained how this is done.
Near the Castle of Erl there lived a lonely witch, on high land near the thunder, which used to roll in Summer along the hills. There she dwelt by herself in a narrow cottage of thatch and roamed the high fields alone to gather the thunderbolts. Of these thunderbolts, that had no earthly forging, were made, with suitable runes, such weapons as had to parry unearthly dangers....
To her he said that the day of his need was come. And she bade him gather thunderbolts in her garden, in the soft earth under her cabbages....
On the grass beside her he laid those strangers to Earth. From wonderful spaces they came to her magical garden, shaken by thunder from paths that we cannot tread; and though not in themselves containing magic were well adapted to carry what magic her runes could give. She laid the thigh-bone of a materialist down, and turned to those stormy wanderers. She arranged them in one straight row by the side of her fire. And over them then she toppled the burning logs and the embers, prodding them down with the ebon stick that is the sceptre of witches, until she had deeply covered those seventeen cousins of Earth that had visited us from their etherial home. She stepped back then from her fire and stretched out her hands, and suddenly blasted it with a frightful rune. The flames leaped up in amazement. And what had been but a lonely fire in the night, with no more mystery than pertains to all such fires, flared suddenly into a thing that wanderers feared.
As the green flames, stung by her runes, leaped up, and the heat of the fire grew intenser, she stepped backwards further and further, and merely uttered her runes a little louder the further she got from the fire. She bade Alveric pile on logs, dark logs of oak that lay there cumbering the heath; and at once, as he dropped them on, the heat licked them up; and the witch went on pronouncing her louder runes, and the flames danced wild and green; and down in the embers the seventeen, whose paths had once crossed Earth's when they wandered free, knew heat again as great as they had known, even on that desperate ride that had brought them here. And when Alveric could no longer come near the fire, and the witch was some yards from it shouting her runes, the magical flames burned all the ashes away and that portent that flared on the hill as suddenly ceased, leaving only a circle that sullenly glowed on the ground, like the evil pool that glares where thermite has burst. And flat in the glow, all liquid still, lay the sword.
The witch approached it and pared its edges with a sword that she drew from her thigh. Then she sat down beside it on the earth and sang to it while it cooled. Not like the runes that enraged the flames was the song she sang to the sword: she whose curses had blasted the fire till it shrivelled big logs of oak crooned now a melody like a wind in summer blowing from wild wood gardens that no man tended, down valleys loved once by children, now lost to them but for dreams, a song of such memories as lurk and hide along the edges of oblivion, now flashing from beautiful years of glimpse of some golden moment, now passing swiftly out of remembrance again, to go back to the shades of oblivion, and leaving on the mind those faintest traces of little shining feet which when dimly perceived by us are called regrets. She sang of old Summer noons in the time of harebells: she sang on that high dark heath a song that seemed so full of mornings and evenings preserved with all their dews by her magical craft from days that had else been lost, that Alveric wondered of each small wandering wing, that her fire had lured from the dusk, if this were the ghost of some day lost to man, called up by the force of her song from times that were fairer. And all the while the unearthly metal grew harder. The white liquid stiffened and turned red. The glow of the red dwindled. And as it cooled it narrowed: little particles came together, little crevices closed: and as they closed they seized the air about them, and with the air they caught the witch's rune, and gripped it and held it forever. And so it was it became a magical sword. And little magic there is in English woods, from the time of anemones to the falling of leaves, that was not in the sword. And little magic there is in southern downs, that only sheep roam over and quiet shepherds, that the sword had not too. And there was scent of thyme in it and sight of lilac, and the chorus of birds that sings before dawn in April, and the deep proud splendour of rhododendrons, and the litheness and laughter of streams, and miles and miles of may. And by the time the sword was black it was all enchanted with magic.
Nobody can tell you about that sword all that there is to be told of it; for those that know of those paths of Space on which its metals once floated, till Earth caught them one by one as she sailed past on her orbit, have little time to waste on such things as magic, and so cannot tell you how the sword was made, and those who know whence poetry is, and the need that man has for song, or know any one of the fifty branches of magic, have little time to waste on such things as science, and so cannot tell you whence its ingredients came. Enough that it was once beyond our Earth and was now here amongst our mundane stones; that it was once but as those stones, and now had something in it such as soft music has; let those that can define it.
And now the witch drew the black blade forth by the hilt, which was thick and on one side rounded, for she had cut a small groove in the soil below the hilt for this purpose, and began to sharpen both sides of the sword by rubbing them with a curious greenish stone, still singing over the sword an eerie song.
Alveric watched her in silence, wondering, not counting time; it may have been for moments, it may have been while the stars went far on their courses. Suddenly she was finished. She stood up with the sword lying on both her hands. She stretched it out curtly to Alveric; he took it, she turned away; and there was a look in her eyes as though she would have kept that sword, or kept Alveric. He turned to pour out his thanks, but she was gone.