You can pick up a Crayfish net at a tackle shop for about 10 lev. They're cylindrical nets wrapped round what looks like a big spring with funnel like entrances at either end.
Attach a bit of string to the net which, in deeper, still water, should be tied to a largish float - plastic bottle. In shallow or moving water, fix the string to the bank somehow. Stick a bit of meat in the net, left over fat/chicken carcass stuff like that, and drop it into the water near some growth. - reeds, etc. Wait a day or two and if there are crays in the water, you'll get some.
If the crays have eggs which you will find on the underside near the tail, replace them in the water. They'll taste crap and you'll also ensure future stocks.
Place your crays in clean water for 24 hours to flush.
To remove the digestive tract prior to cooking, at the tail of the cray there is a fan shaped end made up of what looks like 3 big scales. The middle bit of this fan shaped end is attached to the digestive system. Twist it gently and pull. You should end up with what looks like a bean sprout attached to a scale. Ditch that bit, cook the rest.
To cook, salt up some water and bring to the boil. Drop the crays into the boiling water, add a bit of beer to the water. In less than 10 minutes, you'll have a feast which is a perfect accompaniment to a pint or two of beer.
Be aware, if you have a nickle allergy, you'll get a reaction. Sickness and the trots. Dunno how you know you have a nickle allergy, but you will if you do! I guess.