What do you think of this? = 이것은 어때요?/이것에 대해(서) 어떻게 생각해요? These are polite phrases. More casually is, 이건 어때?/이것에 대해(서) 어떻게 생각해? The structure of "what do you think of __?" is pretty much just (noun) + 은/는 + 어때(요)? Or (noun) + 에 대해(서) 어떻게 생각해(요)?

That is exactly how you'd say it. What do you think of this car? What do you think of this telescope?