“If they give you lined paper, write the other way.”

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

Letter to Denis Diderot, as quoted in The Affairs of Women : A Modern Miscellany (2006) by Colin Bingham

In a letter to her husband Otto Modersohn, from Berlin, 4 February 1901; as quoted in Voicing our visions, -Writings by women artists; ed. Mara R. Witzling, Universe New York, 1991, p. 201
1900 - 1905

Introduction, p. vii.
On Writing Well (Fifth Edition, orig. pub. 1976)

Interview http://www.lordotrings.com/interview.asp with Dennis Gerrolt, first broadcast on the BBC Radio 4 programme "Now Read On" (January 1971)
Context: It gives me great pleasure, a good name. I always in writing start with a name. Give me a name and it produces a story, not the other way about normally.

The Paris Review interview (2010)
Context: Three things are in your head: First, everything you have experienced from the day of your birth until right now. Every single second, every single hour, every single day. Then, how you reacted to those events in the minute of their happening, whether they were disastrous or joyful. Those are two things you have in your mind to give you material. Then, separate from the living experiences are all the art experiences you’ve had, the things you’ve learned from other writers, artists, poets, film directors, and composers. So all of this is in your mind as a fabulous mulch and you have to bring it out. How do you do that? I did it by making lists of nouns and then asking, What does each noun mean? You can go and make up your own list right now and it would be different than mine. The night. The crickets. The train whistle. The basement. The attic. The tennis shoes. The fireworks. All these things are very personal. Then, when you get the list down, you begin to word-associate around it. You ask, Why did I put this word down? What does it mean to me? Why did I put this noun down and not some other word? Do this and you’re on your way to being a good writer. You can’t write for other people. You can’t write for the left or the right, this religion or that religion, or this belief or that belief. You have to write the way you see things.

Source: Life is Elsewhere