In Us, Red says that the Untethering took years to plan, and there are clips of the tethered training for it, gathering scissors, etc.
How were the tethered able to stop doing what their above-ground counterparts were doing and plan a revolt?
The tethered were bodies created that shared the soul and lived underground. There was nothing that robbed them of free will. They could technically do whatever they wanted but didn't because they never had. They have names that are not the same as the people in the world above. In spite of sharing the soul, they exhibit freewill to go ahead and name themselves differently.
Now Adelaide happens to get switched with Red. Given Adelaide is from the world above, she understands free will. Over time she shows the rest of the tethered that they too can decide for themselves. They too can live their own free lives and not merely mimic what the world above is doing.
The un-tethering took time because as a species they needed to be taught how to act on their own free will. The rest of the procurement of scissors and uniform was merely from the perspective of war.