It all depends on what you have available: Metal rings need special tools and a second person to hold the bird, some plastic (spiral) leg bands require at least a second pair of hands. I am using colored zip-ties that i can attach and detach from my ducks without somebody helping me.
Now Mallard ducks: If you raise them, feed and house them i seriously doubt that they would fly away during the cold season. They may look like "wild ducks" but they aren't! Dumping them, for example into a public pond would be as illegal as releasing a domestic breed duck into the wild. They will neither be able to feed themselves nor will they have the instincts to protect them from predators.
Please be responsible and don't do that to any domesticated duck, it is cruel and they will die a horrible death.