“Here is a golden Rule to begin with. Write legibly. The average temper of the human race would be perceptibly sweetened, if everybody obeyed this Rule! A great deal of the bad writing in the world comes simply from writing too quickly.”

Source: Showdown

As quoted in Instructor, Vol. 103 (1992), p. 201

Source: How to Argue and Win Every Time (1995), Ch. 7 : The Power of Words, p. 104
Context: There are no rules that say lawyers cannot write or speak from their heart. Passion has never been formally outlawed, although it is a little-known experience among most lawyers and nearly all academicians.

What I Believe (1938)
Context: I do not believe in Belief. But this is an Age of Faith, and there are so many militant creeds that, in self defence, one has to formulate a creed of one's own. Tolerance, good temper and sympathy are no longer enough in a world where ignorance rules, and Science, which ought to have ruled, plays the pimp. Tolerance, good temper and sympathy — they are what matter really, and if the human race is not to collapse they must come to the front before long.

As quoted in InfoWorld https://books.google.gr/books?id=qjgEAAAAMBAJ&pg=PA49&dq=, Vol. 23, No. 16, 16 April 2001, p. 49. This had been attributed previously to many other sources from 1908 on, according to this analysis https://quoteinvestigator.com/2012/10/22/world-end/ by Quote Investigator.
Misattributed

Source: Snow Falling on Cedars

As quoted in the epigraph in Fahrenheit 451 (1953) by Ray Bradbury; Susie Salmon also uses this quote in The Lovely Bones, and Daniel Quinn published a book in 2007 with the title If They Give You Lined Paper, Write Sideways (2007)
Spanish: "Si os dan papel pautado, escribid por el otro lado" (If they give you lined paper, write on the other side)
"If they give you ruled paper, write the other way" is often attributed to William Carlos Williams who was contemporary with JRJ.
Misattributed