Picture 1′ lie flounder down then carefully open up its gill flap until you see the shank of the hook then gently with a long nose pliers grab the shank of the hook.
Picture 2′ with the shank grabbed gently turn it over the gill towards you and when the shank is out try to hold it with your fingers ,you can put a bit of preacher on their gills but just be careful.
Picture 3′ with the shank healed with your fingers put the pliers into the flounders mouth and grab the curve of the hook. Whit that dun push with your fingers as you are genteelly pulling with the pliers I recommend using barbless or debarbed hooks when fishing for flounder as it helps when unhooking a greedy flounder. Most of the flounder will survive if you take your time unhooking them.