https://medium.com/@theblocknetchannel/understanding-a-decentralized-exchange-eee9e1043f45  heres an article I had bookmarked, hopefully it'll give a good breakdown.. to answer your question specifically though you'd likely have to have an entry point like a purchase of BTC/ETH/whatever pair before going to the exchange, since for an exchange to be decentralized, it would require trading directly from a wallet, and wallets don't support USD/whatever currency.