You have to join a platform that allows margin trading and deposit some bitcoin or whatever coin you want to short into your account as collateral. You can then short or long a position.

Some platforms like bitfinex offer leverage at 1:3. Others like SimpleFX go all the way to 1:500.

Keep in mind that margin trading can generate much larger losses then traditional trading. Although there are margin call limits in place in most cases, it does not mean that you cant lose more money than you actually have in your account.