You can call your broker and say, I'd like to short stock number one and what the broker will do is borrow the shares on your behalf and sell them and then you own negative shares.

Calomiris: I am going to short the stock market big time if the best Bob Shiller can come up with for optimism is the Housing Counseling Office.

It is more questionable when a trader sells a stock short without even taking the trouble to borrow it or make sure it can be borrowed.