To program a human, the person had to be dead. A control was installed in the person, powered by electricity, turning the person into an agent of the Cybermen. The control over programmed humans could be shorted out by another signal, but that killed whatever might be left of the person.
During their race to control the Nemesis statue and conquer Earth in 1988, the Cybermen used two humans, who were controlled by their headsets, to try to eliminate the Seventh Doctor. (TV: Silver Nemesis)
When the advance party of Cybermen from Pete's World entered this universe in 2007, they programmed three Torchwood One employees so they could open the Void and allow the other five million Cybermen onto Earth. The controls resembled two earpieces. (TV: Army of Ghosts)