Regardless if you want to catch crayfish for bait purposes or for consumption, the methods and places to catch them remain the same. Their are several key areas you should focus on to maximize your chance of catching crayfish.
The first place you should concentrate on is man made dams. A lot of dams are constructed of medium sized rock piles. Crayfish love to hide in these rocks. You have 2 options in this location to catch crayfish. You can flip rocks over and catch them with your hand, or set a trap out and tie it to a rock. Both methods will catch crayfish. The difference is how quickly, and how much effort do you want to put into it. Catching them by hand will instantly get you a few crayfish, while setting a trap will take hours, or possibly and overnight stay. You are more likely to catch a larger amount using a trap but, will take longer and the trap could be lost of stolen.
Boat ramps often offer a place to find a small number of crayfish. Most boat ramps in Texas are lined with the same rocks they build dams with. Simply flip over rocks and catch the crayfish by hand. I wouldn't recommend setting any traps in this location. A boat is likely to run over it or if could even be against the law. Most boat ramps say you can't fish within a certain amount of feet. Setting a trap for crayfish may be considered fishing.
Small ponds can sometimes offer a large number of smaller crayfish. The method for catching crayfish here is a little different. Small ponds do not usually offer many rocks. You can set a trap or fish for the crayfish. Yes I did say fish. Get a 10 foot piece of fishing line, a couple of screws or nuts, and some bacon. Tie the nuts about 12 inches from the end and then attach the bacon to the same end. Tie the other end of the fishing line to your arm. Toss the bacon and nuts out a few feet and let them sit for a few minutes. Slowly drag the bacon back to shore. If you are lucky, a couple of small crayfish will be attached to the bacon eating it. With a net scoop them up. This is by far the fastest way to catch large numbers are crayfish. Unfortunately finding a pond that has a large population of crayfish is hard to do. Also this crayfish seem to be small in size and not really worthy of eating. This would only be good for using as bait.