CO2 Method:

Materials:
-Co2 tank (around 18dollars at paintball shop, depending on size, I use 16oz, about $3 to fill, you don't need to fill it often)
-Air tight container, sized to fit feeder(s)
-surgical tubing
-glass of water
-bedding/papertowels

Setup/How this works:
Run tubing from the tank to a hole near the bottom of your container, make sure it's a good seal. Run another tube from a hole in the top container out into the glass of water. Since Co2 is heavier than air it fills up from the bottom pushing air out the top tube, the glass of water make sure air can get out but not back in.

Place your feeder(s) in the container and seal it shut. Turn the gas on low, it acts as a sleeping gass so they fall asleep, when they've stoped moving turn up the gas to remove the rest of the air from the container.

Turn off the gas (or leave on low if your not sure how much you needed) and wait (I usually go 5mins to be safe). Make sure the gas is off, and try not to breath in the Co2. You can now remove your freshly killed rodents. No stress, no trama, just sleep then nothing.

The bedding/paper towels we're for the bottom of the container. Sometimes rodents defecate when they die so you'll want to put something in there to catch any possible mess.

Other Co2 methods:
The sublimation of dry ice creates Co2. Place ice in small tupperware with 'air' holes. Place tupperware in container with rodents, seal, and wait. Do NOT put dry ice in loose with rodents, it 'burns' them on contact.

Combining vinegar and baking soda also creates Co2, I've never used this method cause it's smelly.