You punish the dissidents and reward the obedient, and you make everything simple and absolutely black and white, so that the morality of any given position or policy is inherent, and unquestionable. It becomes painful for the thinkers to think in public, and very easy for people to choose the path of intellectual torpor and compliance.
These wheels, once set in motion, require very little effort. The earliest Soviet propagandists/information warriors, started their work in this country in the 1920’s.
And the same thing applies to supposed intellectuals. Even they in all too many cases take the easy routes. The, as they supposed, dead ends they hit in philosophy, and the metaphysical pessimism necessitated by their on-going embrace of 19th century materialism, made any and all easy routes all too attractive.
That is all Leftism is: the easy route. The path which foregoes thinking, the path which not only encourages groupthink, but more or less demands it. You get tests where most of the answers are already filled in., and you get membership in a marvelous club which regularly regales you with praise as to how intelligent, insightful, and HUMANE you are. And you get to indulge the hate which your protean self makes necessary regularly and to your fill, without guilt, and without any need to hide it.