The “proper” way to trade currencies is through a dedicated currency account set up with a FX broker. Alternatively, one could trade 'em through discounters such as Schwab, TD, IB, etc. in mini lots. Yet still more alternatively, one could use open-end mutual funds or ETFs/ETNs.
For all the rumors of its immanent demise, the $US is still the “big dog” among the fiat currencies, with gold/silver remaining the only “true money”.
If you choose to go the mutual fund route, both Rydex and PowerShares offer a way to trade the $US long or short. Initial purchase mins vary from broker to broker. But thru Schwab, it’s possible to trade RDPIX/FDPIX or RYSBX./RYWBX with a $100 min and no ST trading fees.
If you choose to go the ETF/ETN route, more currencies become available to trade: the Euro, the Yen, the Pound, the Kiwi, the Franc, the Loonie, and the Yuan. Additionally, some of the more popular cryptos become available. But if you go the ETF route, do two further things. #1 Check to see how liquid the contracts are and how tight the bid/ask spreads are. #2 Run a correlation matrix on your choices and rejections. What you’ll find is that the $US and the Euro are pretty much inverses of each other. Hence, there’s no need to trade both of them. I.e., to go long the $US is to short the Euro, and vice versa. So pick contracts that have good liquidity and tight spreads. Also, gold/silver tend to trade inversely to the $US.
Now for some cautions. Open-end mutual funds price once a day, at end of day, and can’t be trailed with a stop, nor traded OCO. OTOH, if you’re a late riser and if you trade more on fundamentals than technicals, open-end funds can be the perfect match.
In short, “Chef’s Choice” as to which vehicles to use, as well as whether to trade currencies at all. I’d argue, though, that at least the major pairs should be tracked. What happens to the $US has a very direct impact on your life. As its price rises or falls, the goods and services you buy become more or less expensive. You can’t control prices, but you can respond to them and --hopefully-- can profit from their changes.
Standard Disclaimers: I’m currently long the $US with a tiny position.