I catch them with a dip net. I find weedy roadside ditches with water or shallow, weedy shorlines and drag a minnow net through the weeds. I can catch hundreds that way. But I only use them for bait. If you're after crawdads for eating, a roadside ditch probably isn't your best source.
I took my two oldest boys catching crawfish for bait this summer(6 and 5 years old) with a dip net in a mudhole. Before it was over my 5yo was wallowing in the mud like a pig. They had a blast and I was glad my Toyota is 17 years old and nasty already. That is one of the regrets I will have selling her to get a new truck, we cannot hurt it.
actually scrolling thru an isle at work on thursday and saw that we (sportsmans warehouse) sell crawfish traps! thats cool wouldnt think they had em but we do!! not that i go after em but ill tear the arse end outta em!!