What does the quote "If they give you ruled paper write the other way" mean and how does it relate to Fahrenheit 451?

It is attacking the fact that there is completely no individuality in terms of schools of thought and anyone and everyone is suppose to conform. The quote is interpreted in the sense that you should not follow rules,  or norms in a larger sense bluntly and blindly, or maybe even to the extent of laws. Rules, in the finest sense are meant to be broken. Mildred, for example, does not find any wrong in keeping a book in the house, and her thoughts are different from Clarisse, Montag or Faber. Due to Clarisse's influence, Montag is convinced regarding the err of his ways and he breaks the rules, laws, norms, trends that he used to follow.