It is better to search flounder fish in the evening or morning time. These are two times when they are in shallow waters, while you can look for them in deep water when the day is full bright and sunny.
Flounder are flat fish with eyes on top, and they swim in separation, not in schools. Also they tend to swim from bays and channels towards see water. You should mark the search points for flounder accordingly.
To fish for flounder use chartreuse spoons. They are the best lures for flounder in bays and gulf areas. The flounder tend to hang longer on the spoons and the success ratio of using spoon is greater than other lures.
You can also use pounders, the soft plastics on jig heads. Just throw it in the water, let them dip down in the deep and then jerk it. After the jerk let the plastics hit the bottom again. This bump will lure the fish and they are more tend to ambush it.
The best way is to lure wish with live shrimp, croakers or any other such stuff. They are more likely to attract flounder for ambush than the artificial or plastic lures. The real food's smell is a trap that attracts flounder.
Fishing flounder consumes a lot of time. Be patient and let the lure sit in the water for a while, and also jerk it after every now and then. If you use real food, it is likely it will consume less time, while artificial lures can take more time.
Sometimes success ratio is almost nil, so be patient. Flounder is so much patient fish that it might hang around with your pole longer than you expect. So use their tactics, instead of taking out pole and lures after every now and then. This will scare away the fish.