To do it, you need to know three things: grammar (how language is used to express thought), logic (how arguments are constructed), and rhetoric (how arguments are presented).
Not quite...
The things you read or hear have to be TRUE!!
If all the things we read or heard were true, we wouldn't have to worry about critical thinking.
But in *this* world, we're being lied to from all sides. Besides, most lies are mostly truth anyway, otherwise they wouldn't be dangerous.
Critical thinking, in its proper form (not the "pasteurized process cheese food product substance substitute" that modern educationists are hawking under that label) is the tool with which you sift out the lies embedded inside the "mostly truth" that you read or hear.