Catching crayfish for smallie bait is / was one of my specialities back in my younger days!
Take a seine net, these can be purchased in most sporting good stores, and grab a buddy or two. This can be accomplished alone but a second guy to help is a HUGE plus.
Find a shallow, rocky stretch or stream or river, 2′ deep at the most. Shallow swift water where lots of rocks the size of a basket ball and smaller can be found is perfect.
Take the seine and string it across the creek at the bottom end of the shallow, fast water. You may have to use some rocks to pile up around the posts of the seine to get it to stay upright. Don’t string it up tight… allow a “belly” to develope in the middle of the seine.
Walk back to the top of the shallow run and begin to cut a zig zag pattern from side to side as you work your way back dwon towards the seine. Shuffle your feet and disturb every rock as you go… or at least as many as you can. No need to hurry. All you want to do is disturb the crayfish from under the rocks and get them out into the current. When a crayfish is disturbed in fast water he scoots off into the current and lets the fast water wash him out of harms way… right into your awaiting seine net.
Keep working that side to side shuffle until you get down to your seine. At this time that second guy comes in handy as he can help you left the seine without spilling the crays. And you will have plenty! Often dozens – some times hundreds. Of all sizes and in various stages of molt.
If this was any easier or more effective it would be illegal.