When you know for yourselves that, "These qualities are unskillful; these qualities are blameworthy; these qualities are criticized by the wise; these qualities, when adopted & carried out, lead to harm & to suffering" — then you should abandon them.

When you know for yourselves that, 'These qualities are skillful; these qualities are blameless; these qualities are praised by the wise; these qualities, when adopted & carried out, lead to welfare & to happiness' — then you should enter & remain in them.

Selected part from the Kalama Sutta. Seems like a good rule of thumb; combining virtue ethics, deontology, and consequentialism. This would be my first instrumental rationality lesson for the low-IQ people. The rest of the lesson would be specific examples and discussion.