Off a sandy bottom with a hook, a shrimp, and a boat. Serious. Incredible fish, I'd love to have one in a SW tank. I live on the gulf coast, I've always been intrigued by flounder.
Same for me, bought 1 many years ago (listed as freshwater) put it in the tank, where it immediately disappeared, until found it dead a few days later.
Key is to determine whether or not you're getting a hogchoker (not true FW flounder) or a true Peruvian FW flounder. Hogchoker's are easy to keep but you need a gravity of about 1.012 or greater (almost to full salt). I know a guy who catches the chokers in his cast net by my beach house. He has a tank setup that he uses the lagoon water from behind his house to do water changes. It's not full salt. Probably around 1.012-1.015. He's had them for a while.
The only true FW flounder is the Peruvian variety.