[KarenD]

All of the W- adverbs can be used to introduce subordinate clauses. It's helpful to think of them as abbreviated forms of Noun Phrase + Preposition + Relative Clause: "how" is the same as "the manner/way in which", "when" is the same as "the day on which/the time at which", etc.

So they are adverbs functioning as subordinators.

This is how I make the soup = This is the way in which I make the soup