I don't think you can term flat-earthers intelligent. That's a dangerously low bar.
> A good method for this is: pick an idea that you are 100% sure is true. Not "the Earth is round" but "X value is good." Then search for people who disagree.
This is a neat trick (does it have name?), but the true value is in realising their fallacies to hone your own thought process, not why / how they convinced themselves through deliberation, and contemplation, and divine knowledge, and self-enlightenment, and what have you.
> Study philosophy. Study formal logic.
Philosophy ranked as the highest form of knowledge among the polymaths in the Islamic Golden Age: https://en.wikipedia.org/wiki/Falsafa
The encyclopedia of Ikhwan al-Safa and other such massive bodies of work are representative of Philosophy's central role among the scholars at the time: https://plato.stanford.edu/entries/ikhwan-al-safa/