Here's how I used to trap leeches. Get yourself a large soup can or coffee can, toss in some chicken liver, a couple of rocks and crimp the top closed. Leave a few spots uncrimped for the leeches to get in.
Now find yourself a shallow pond with leeches, preferrabally one that winter kills and has no predatory fish in it. Place the can in the pond just before dark and make sure you remove it before the sun comes up. Leeches feed at night and
will leave the can when the sun comes up.
Remember, adult leeches die after spawning so you will only have a limited time to catch them. Once the water warms too much they die. You still have enough time this season. Good luck.