I did not get an answer, so I will assume I would have unlimited resources.
Everything has to happen young on.
One of the first step would be isolating smart people from the others. From early on, every guy with over 90+ IQ to private boys-only school. No home
vacation, no sports facilities, TV till age 21. The social isolation, no experience with opposite sex would create very socially awkward people.
At the same time, massive campaigns on creating the image of the smart guys being very uncool. Every loser, every bad guy in movies would be some
supernerd. Create an image of smart people always losing, while dumb superheros/main characters always winning thanks to their no-thinking attitude
and pure strength. Such guys would get all the riches. Every popular series, popular trend would focus on dumbness as a virtue, so making sure
dumbness is valued the most
Similarly I would make sure that certain characteristics like pure strength, fashion sense, status,intolerance and money would be valued even more
than it is right now.
Make sure that dumbness is awarded. The dumber you are, the more you will get rewarded by the society. Fame, riches come to you if you are dumb. That
would be easily done with manipulating music/TV industries, ignoring every success story of not-dumb people.
"Poison" the foods, drinks with IQ-lowering chemicals.
These were the first ideas that came to mind
I would write more, although currently I g2g, maybe Ill add something later.
I guess you are trying to pick out the ideas here and use these to compare to real-life, what is happening now, right?