How did the numenorians and dwarves craft items of magical properties?

[Rumil]

Indeed, welcome to the old Barrow Paradus,

Numenoreans, or more accurately their descendants in Arnor, made magic weapons, think of the Hobbits' Barrow swords. Merry's sword was iirc made with spells specifically to destroy the Witch-King.

I vaguely remember the Dwarves being described as casting spells over the magic weapons they made, probably in one of the poems?

Beorn was said to be 'something of a sorceror' by JRRT in one of the letters. It seems that his shape changing was a 'magic' ability but a very specific one. Also the men who became the Nazgul were kings and sorcerors prior to their Nazgulification. Note JRRT doesn't say Wizards here, they aren't Maiair! Possibly the Mouth had some sort of 'magic' - or 'deceits of the enemy' as Galadriel says, but he's been the subject of much debate and clear conclusions haven't really been reached.

I get the impression that men's magical ability was either a specific (possibly inherited?) trait, like Beorn's shapechanging, or the product of some talent and much study (the 'sorcerors' and weapon smiths) but very rare and never as powerful as the elves' natural talents.