Thank you very much!
Just now I asked a question: Is this sentence How does a small child do? correct? You told me it's wrong. So I asked this question: Are these sentences correct? You told me all of three are correct.
How do you do is correct? Why is How does a small child do? wrong?
Because "How do you do" is an idiom used only in one very specific and fixed application. You cannot play with any part of it.
And Mr Raymat, Are my explaination correct？
How do you do? means How are you?
What do you do? means What are you? or What's your job?
How do you do it? mean what ways will you use to do something?