What do you do with them once you have trapped them? Does throwing them into a hessian sack and in the dam still work 
Or a crab pot

They do say to be humane about it as the links which Beatle gave.
What a time consuming task that is to follow the written instructions and build the twin cage as per the link I gave above. There is room for improvement in the way the instructions are written, so take your time with them.
If your thinking of building one your self, I recon, look for an old bird cage at a garage sale and modify it it, saves a few $ and a lot of time. The main thing to achieve is the method of encouraging the birds into the cage, and then to be trapped and want to go up high into the 2nd holding section. If the funnel, and valve arrangement is made so that they can't go back, it will be good.
The bait game has started, then this week when kids are at school, I shall try the cage out.