The same way Hitler did it - Massive bureaucracy, declaring large swaths as non-persons. Much of it was done indirectly - The upper bounds of those numbers attribute certain large famines as being politically motivated or blame Stalin for deaths caused by mismanagement, but even conservatively, he ordered millions of people forcibly resettled into areas that were not productive, and then failed to feed them. He sent hundreds of thousands to prisons in Siberia for political crimes - Not personally, but by setting up the system so that those were crimes and the whims of his secret police were followed.

Hitler never killed anyone - If you're talking about holding the gun. Hilter still had blood on his hands, and Stalin had more.