How Are Words Added to the Dictionary?

Last month, Merriam-Webster announced that 530 new words have been added to the dictionary. Among them are new meanings of existing words, such as “they” as a nonbinary pronoun, “free solo” (as a climber, this is particularly exciting to me), “Bechdel Test,” and “dad joke.”

How, exactly, are these new words added? What criteria does a word have to meet in order to find its way into the Book of English Words?