Learn French: Elle sait mieux couper les cheveux blonds. - She knows how to cut blond hair better.

mieux is the comparative form of the adverb bien (well), meaning "better". It describes how she performs the action of cutting. meilleur is an adjective used to describe nouns, so you would use it for something like "a better haircut".

Why does mieux come right before couper?

When a short adverb like mieux modifies an unconjugated verb (an infinitive like couper), it usually goes right before that verb. That is why it is mieux couper (to cut better).

The English translation says "knows how to", but the French sentence just says sait. Where is the "how to"?

In French, you do not need extra words for "how to". The verb savoir (to know) followed directly by an infinitive verb automatically means "knowing how to do" that action.

Why is les cheveux blonds plural when we say "blond hair" in English?

In English, "hair" is usually an uncountable singular word. But in French, the hair on your head is always plural: les cheveux. Because the noun is plural, the adjective describing it also needs to be plural, which is why blonds ends with an s.