Flatfish need very small baits, this image shows the typical bait. The rag worm is small or often cut in half, you dont need it trailing off the end of the hook as the fish wont get hooked that way.
The bait is tipped with a tiny slice of squid to hold the worm on.
Baiting up this way often means the bait can be reused as it rarely comes off when you have a fish. This comes in handy sometimes, especially when you are far from home and the fish are feeding!