What tackle to use
For flats fishing a 9 - 10wt outfit, intermediate lines with 9 - 12ft fluorocarbon leader, large arbour reel holding plenty of backing.
For offshore the same setup but using a quick sinking line or shooting head.
What flies to use
Clousers; Crab fly; Deceiver; Prawn imitations; Poppers
On the flats a yellow over white clouser matches the small whiting they love. But crabs, shrimps, deceivers and most flies can also work. Size 1/0 to 2/0 is about right. Approach cautiously when up on the flats - they can be tailing or cruising - but when they are up on the flats and undisturbed they are in eating mode and are very alert.
Present the fly ahead of the fish and begin stripping as the fish gets closer. Sometimes they can be found in the presence of rays so keep an eye out for that too. They have a soft mouth so treat with utmost respect while releasing.
While casting to feeding fish on the flats, a minimum number of false casts with an accurate presentation of the fly is very important. Once spooked, they will make their way off the flat and not eat. If you're lucky you may get multiple shots at the same fish, it really depends where you are.