First, before anything else, you must have a good reason to short any currency. The better your reasons, the lower the risk and the higher the return.
Once you have the idea, now you need some money. The more you can get the lower the commission percentages, so investing too little may not be a good idea. You should get at least 200 dollars. Another possibility is to borrow dollars. Borrowing dollars that you don't have is of course riskier than using your own, since you have to pay them back.
The easiest way to short the dollar is to simply drive to a currency exchange business (like those found in airports), and trade your dollars for another currency. Once the dollar has gone down you can then trade that currency back to dollars for a profit. This is probably the simplest and fastest way to profit from a fall in the dollar' s value, but it may not be the most practical one if you plan on trading the dollar often.
Another way to short the dollar is to open a FOREX account and but other currencies in that account. This is practically the same thing as above, but once you have a forex account you can short the dollar from any computer and change it for most currencies in the world! Furthermore, conversion commissions may be much lower.
Thats it. Remember to check the warnings and tips sections of the article. There's key information there!