First of all, you would join the youth wing of the Communist Party. In some times and places this was basically mandatory, but even when so, it was fairly obvious who took it seriously enough to plan for a political career. You would volunteer for leadership roles in the Komsomol or equivalent in other countries, and attempt to curry favor with people in power. Communist apparatchiks came from both industrial/agricultural Party organizations, and academia. Communist theory was supposed to favor people with proletarian origins; like any theory this was more true in some times and places than in others. By the last decades of the Soviet Union, the idea of a leader of humble origins was unusual enough for Mikhail Gorbachev's peasant childhood and early work in a factory to be widely remarked upon.