How do you short a currency?

There are basically 3 ways to short a currency:

1. You can sell a contract for that currency in a Futures Market.

You can sell an index of that currency. For example, if you believe
that the US dollar is going to weaken, you can sell a US dollar index,
which measures the value of the USD against a basket of currencies: U.S. Dollar Index

3. You can sell that currency against another currency in the Forex market.

Forex all currencies are traded in pairs. Each pair’s name is an
abbreviation of the names of its currencies. There are many different
pairs but the main trading activity is concentrated in the following:

EUR/USD  –  Euro versus US dollar

GBP/USD  –  British Pound versus US dollar

USD/JPY  –  US dollar versus Japanese yen

USD/CHF  –  US dollar versus Swiss franc

USD/CAD  –  US dollar versus Canadian dollar

AUD/USD  –  Australian dollar versus US dollar

NZD/USD  –  New Zealand dollar versus US dollar

There are also pairs called crosses such as:

EUR/JPY  –  Euro versus Japanese yen

EUR/GBP  –  Euro versus British pound

EUR/CHF  –  Euro versus Swiss franc

The list goes on.

price of the pair states how much the first currency is worth in the
second currency denomination. For example, if the price for EUR/USD is
1.3500, this means that one Euro is currently worth US$1.35.

are always long or short one side of the pair against the other side of
that pair. When you buy the EUR/USD pair this means that you
simultaneously buy the Euro and sell the USD. When you short the Euro,
you basically sell the Euro and buy the USD.