Very carefully.

Peetee has the best idea of using a trap, but you can make one yourself with things you probably have on hand.

Here is an idea of how to make one yourself:
http://www.coralcorral.com/id19.html

You can also just use a jar, bottle, kritter keeper, or anything you have on hand laid on its side with bait in it and a string that allows you to pull it upright and out of the water (you don't want to put your hands in a tank with an upset mantis!). You should remember that these have the ability to break glass, so use something made of plastic.