Catching any kind of fish begins with putting baits where the fish are. For catfish, which feed mostly by smell and taste, it is all about using the right kind of bait.
Of course, different species and sizes of cats prefer different kinds of meals, and some offerings lend themselves better to specific styles of catfishing than do others. Also, catfish are just like other kinds of fish -- or us for that matter -- in the sense that their preferences vary from day to day. One day's hot bait commonly may not yield much the next day, with no obvious change in conditions. With that in mind, you are wise to set the table with at least a couple of different kinds of baits and allow the cats to dictate their preferences.
Despite the never-ending list of options available to catfish anglers, certain baits do tend to outshine the rest. Big catfish like big meals, and few things do more to improve an angler's odds of landing a true trophy cat than baiting up with a big chunk of cut shad or even a live shad. Adult flathead catfish feed almost exclusively on live fish, and shad often are an important part of the mix because they are around river channels, where flatheads spend the most time. Even channel catfish, which feed on a little bit of everything both dead and alive, turn heavily to fish diets once they get larger than 10 pounds or so.
Shad make great bait in most reservoirs and many rivers, because they are prevalent natural forage and often are readily available to fishermen. However, the same principles apply to various minnows and other baitfish species in waterways where shad are not the main attraction.
Generally speaking, shad should be cut into chunks or strips, with the size of the pieces and the type of cut determined by the size of cats being targeted and the size of the shad. Probably the most efficient way to cut up a shad is to slice off the head and tail and cut across the body to create strips. If those pieces seem too large, the strips can then be cut in half. However, some anglers prefer to fillet large shad and cut up the fillets or to fish with very small shad, either whole or cut in half.
You also should not overlook a large shad's "guts." The entrails produce a very strong smell and often attract strikes almost immediately. Shad guts do not tend to yield as many large fish as do chunks of fillets, but they get a rod tip dancing and definitely are worth putting out there for the cats to consider.
Cut shad can be fished several different ways for good success. The most popular technique is to use a Carolina rig with a large enough barrel weight to keep the rig on the bottom. Lines are spread around the boat or along the shore in a range of depths for lake fishing, or they are cast downstream, often from the head of hole, in rivers. This simple approach is tough to beat any time the cats are using known locations such as river-bend holes or the sides of humps. If the fish are more widespread or their locations are less certain, a good alternative is to drift, using either three-way rigs or bottom-bumping rigs.
For flathead catfish, shad need to be alive and at least palm-sized. Top-end predators, flatheads have minimal interest in cut bait. Baits should be presented on or near the bottom around good structure and thick cover. In rivers, the flatheads are around outside bends among tangles of tree branches near the bank. In reservoirs they typically are along bends in the old creek or river channel or near channel confluences.