To stun them you can put them in a bag and bash them against something hard like the floor. You can also place a firm object like a ruler at the place where the spinal cord meets the skull and give a quick upward pull to their tail, separating the two.

I don't condone any of these methods, they're just ways I've heard to do it. I've also heard that stunned rats that wake up mid coil are more prone to put up a fight.

And the most humane way to prepare fresh killed prey is to put them to sleep with CO2 and then after they're asleep crank it way up to kill them.

Dennis