First off, they're called Crawdads in my neck of the woods. And all you gotta do to catch them is google the design for a 2-liter bottle fish trap, and then make one, bait it with a dead fish, old sandwich meat, or dog-food, and toss it in a creek.
I have kept them fairly extensively, in several different size tanks. My favorite of which is 55 gallon, home to a yellow bullhead catfish, a LMB, and an Eastern River Cooter. In there I keep 3 two inch crawdads peacefully, but have kept larger specimens and more of them back when my turtle was smaller. The key to keeping crawdads\crayfish happy is plenty of hiding places. Several spots to hide for each one, but they may still fight a little.