(1) Assume true what you think is false, (2) try to show something crazy follows from what you assumed, and (3) conclude that what you assumed to be false is actually true.
Assume the moon is an optical illusion. If the moon is an optical illusion, then no one can land on the moon. So no one can land on the moon. But that's crazy: Neil Armstrong landed on the moon. Therefore, the moon is not an optical illusion.