I know this is going to sound really stupid. But I am responding to a twenty day summons. And it says "you must respond to the complaint by stating your defense in writing, and serve a copy upon the court and the person signing this summons, within twenty days." How? Mail? Deliver in person?

You could mail it (and I'd get a delivery receipt) OR hand deliver it (in which case, make sure you have a copy with you which the opposition will date stamp, same with the Court).

If you have been served with a Summons there is no longer a need for personal service of what are basically follow-up papers.

Thank you, I did mail it.