what is the meaning of "supposed to do" in
"How do you supposed to do that?"

See a translation

"supposed to" has a similar meaning to "have to/should do"

Actually, if I translate it to Korean, they might be exactly the same.

I'm supposed to work today. / I have to work today. (Same meaning)

In your sentence, it can be used to mean someone doesn't know how to do something, and so they are asking how they should do something that someone asked them to do. It has a negative feeling.

It's the same as saying "How should I do that?" but it also means that it might be too hard for them to do, so they are shocked someone asked them to do something they can't do.

Suppose라는 단어는 여러가지 의미들이 있으니까 딱 정확하게 설명하기가 좀 어려운데요. 이 링크가 도움이 될 것 같아요 ^^

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