1) borrow the stock

2) sell it

3) rebuy it at the lower price (assuming you're right)

4) give it back to whomever you borrowed it from plus a consideration for letting you hold what's theirs for a bit

Whatever's left after you return the stock and pay the interest is your profit, which comes from the people who bought it from you in step 2. If you're wrong, and the price goes up, you have to replace the stock you borrowed at a higher price than you got for it and that's your loss (which could potentially be infinite, as opposed to long positions where you can only lose what you initially invested)