There are plenty of reasons to short a stock and not all of them...

There are plenty of reasons to short a stock and not all of them involve betting against the company. Hedging a convertible bond to remove the option bit is an example. Generally though, a speculative bet is for the stock price to fall and to buy back in at a later date, hopefully at a lower price and make money after all costs.

Two main ways to go short are physical and synthetic. Physical involves borrowing the stock and selling it on market. In practice, the shorter doesn't take possession for very long, instead the delivery of the stock is taken care if by their lending agent (an investment bank normally). On settlement day, the lending agent sends the stock to the buyer and the shorter gets the money... but they must lodge collateral with the agent.

The other way is synthetic, where the trade is executed with their prime broker. Loosely speaking a PB is both custodian and lending agent for long and short positions. The hedge fund (shorter) sells the stock short and the PB takes care of everything else. In both cases, the shorter must ensure that the borrow is available by either checking before trading or by having the PB provide them with a list of guaranteed available stocks with a fee schedule.

Note that there is no magic wand that makes the stock price fall _after_ the short is put on. If the news flow is bad, or the sentiment is bad, then the position is more likely to be profitable... if not, then tough luck shorter.

I hope that helps.