“He wanted paper and something to write with, some way to sustain a thought, to place it in the world.”

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

Banksyhttp://newsvote.bbc.co.uk/1/hi/entertainment/arts/2018739.stm
Variant: Some people want to make the world a better place. I just wanna make the world a better-looking place. If you don't like it, you can paint over it!
Source: Wall and Piece (2005)

Misattributed
Variant: If they give you ruled paper, write the other way.
Source: Epigraph, in Fahrenheit 451 a translation of a statement by Juan Ramón Jiménez

1949 - 1958, Speech to the Penguins' (1949)

"One Day in the Afternoon of the World" (1964)
Context: I began to write in the first place because I expected everything to change, and I wanted to have things in writing the way they had been. Just a little things, of course. A little of my little.

The Great Master of Thought (Amen- Vol.3), Observing management