처럼 is a postposition meaning "like, as", so u can use it after a noun and add a verb: 아이처럼 웃네요! "(He) laughs like a child!" 같다 is a verb meaning "to be/look like", so u can use it AS a verb: 나 바보같지? "I look like a fool, dont I?" 만큼, meaning "like, as, to the extent of" is much more linked to degree / quantity, either as a postposition (after nouns), or as a "dependent noun" (as the head of attributive clauses): 꽃만큼 예쁘지! "She's as pretty as a flower" 그만큼 "that much" 일한 만큼 받다 "get paid for the work done" According to the dictionary, both are fine in the sense of "as" like in the following sentence (seems to me that 처럼 is more common): 눈처럼/만큼 희다 be as white as snow