As has been said you will not be able to see Flounder/Fluke on the fish finder but you will find structure for fishing.
Structure is a simple as a 2' rise in the bottom, channel edges, creek bottoms, wreck, obstructions, etc. These all hold bait which inturn hold fish.
As anything practice with your finder and you will be come quit adapt at reading the bottom, fish marks, bait balls, etc.
Best way to figure out where you are going to fish is to ride over the area and mark on your GPS the points of structure, your GPS unit will also show your drift line.
Pretty soon you will have a book full of numbers (longitude and Latitude) where you have found some good structure.
A lot of structure has been found by accident going from place to place, it is good to have a high speed transducer which will read while you are motoring.
If you are going to get serious, the use of your charts, GPS and Fish Finder will put you where you want to go.......as opposed to just following the fleet around and hoping they know what they are doing.
Prime example is one of the more favorite channels that the fleet fishes for Fluke in consistantly holds more fish at a certain depth line. Many times I have been out holding this line having my people bail in good fish while other boats were 50-60 feet away not catching...at times it is just a matter of a few feet.......good luck, anything we can do to help