He started even before he became the leader of the Soviet Union by placing his own people in positions of power, as well as eliminating the opposition so that there would be no one to oppose to his dictatorship. When Lenin died, he made sure his only real threat to take the throne from him, Trotskij, was eliminated, and then he prosecuted everyone else who wasn't on his side. He executed about 90% of the Bolsjeviks who had stood by Lenin during the revolution!