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.

你可以打电话给你的经纪人说，我要做空一号股票,经纪人就会在你的名下赊入这些股票,然后卖掉,这样你就拥有数量为负的股票了。

耶鲁公开课 - 金融市场课程节选