They kill like most other predatory mammals. A bite to the back of the neck is usually what happens. The bite itself is just about an instant kill for the mice. The ferrets need to do to a little more maneuvering for the rats and other larger prey in order to get into the perfect position for a neck bite.
When they're in a group, they'll also fight over prey.
I'll have some videos of them killing mice/rats/duck. I'll see if I could upload them.