Okay take your good ol 2L soda bottle and make sure its washed like its never been washed before. And remove the label. Okay now make yur cut above where the label was, right where it's starting to taper up to the "cap section". Just make your cut all the way around. remove cap. Now just stuff the "cap section" upside down into the "bottom section." It should look a bit like a half a bottle with a funnel stuck in it. If the cut was just right, the funnel might just wedge in the bottom section and stay. If not, or if you want it to really stay, you can use a variety of non-toxic water-proof glues. Or duct tape. Handy man's secret weapon. I like to tie a long string around the cap section so i can yank the trap out faster.
You have to take into consideration the size of the mantis. ie. will the thing fit into the trap? If not, just cut enough of the top away so that it will fit but not so big that the mantis will fin it easy to get out.
Now just weigh it down with some pebbles or something and bait it with the mantis's fave food whatever that might be. Put it in and check it periodically day/night. it may take a while. If it takes too long, don't let the bait lose its smell or decompose. replace it and throw the trap back in.