"Not at all." You can also use "Go ahead." or "Be my guest." but they may depend on the specific situation.

"Not at all" is the first response I can think of. If you want to be absolutely clear, then use "No, I don't mind". You can technically say "yes, I do mind", but this is a little blunt and unfriendly for native speakers to hear. We're usually more diplomatic, so we use a phrase to introduce bad news, then give a reason. We don't usually reject the request directly unless we are quite angry. You've probably come across the "buffer" phrases already: "I'm sorry, but...", "I'm afraid that...", "Unfortunately, ..." and so on. Do you mind of I sit here? I'm sorry, but my friend is sitting there. She's just stepped out for a second. Do you mind if I smoke? I'm afraid that I'm eating at the moment. Do you mind paying for dinner? Unfortunately, I was about to ask you the same thing!

"Do you mind..." really means, "Is this a problem for you?" So you would say, "No" if it is not a problem, and "Yes" if it is a problem.

When you don't mind: "No, I don't mind at all!" When you DO mind: "Yes, I do mind."

Hi Someone would usually say, Do you mind if/that - Do you mind if you go home as I am tired? You would say. I don't mind, or That's OK, That's fine. I understand, I'll go now. If you did mind, you would say Yes I do mind actually. or I don't want to go home as I came all the way here to see you! If you were very annoyed you would say. Yes I do mind! I'm not going now. Do you mind that there's only one potato and not two potatoes as I promised? No, I don't mind. That's fine. It's OK. or I do mind, give me what you promised. I'm not happy about that, no. Do you mind me jumping the queue? No that's fine. or I don't mind, it's OK. Yes I do mind! I'm not moving.