Answer it in any way you would answer "How are you?" It's the same statement with an active voice, <<do you do?>> instead of <<are you doing?>> Another analog is <<How do you fare?>> (from old-world "How farest thou?" and so on) but no one ever says that.
You can say "Wonderful, and you?" and I don't think anyone would bat an eye. This is only my take, from personal experience as a native.
It would feel odd if someone said "How do you do?" to me, and I didn't respond.. I'd feel compelled to respond.
"How do you do?" probably makes everyone a little nervous. I know it does me. You're totally normal.![]()