How do you teach a child to name their emotions?

Mostly by narrating rather than instructing. Name what you see in the moment — 'you look really frustrated that the tower fell' — without immediately trying to fix it, which teaches the vocabulary and that the feeling is acceptable. Name your own too, in age-appropriate terms, since children learn regulation largely by watching adults do it. Books and stories give useful practice at a safe distance. Expect the skill to appear long after the vocabulary does; a four-year-old who can say 'I am angry' will still frequently hit first, and that gap is normal rather than a failure.