You get a margin account from your broker and request permission for shorting... then open a position by selling x number of shares of the stock (assuming the s

You get a margin account from your broker and request permission for shorting... then open a position by selling x number of shares of the stock (assuming the stock is available to short). You make sure you have enough $$$ in your account so that in the event of the stock price going up, you aren't forced to sell at a a loss. Grossly oversimplifying things, rather than short, you can also buy put options on the stock which will increase in value if the stock goes down. Taking an options position is far more time sensitive than an outright short.