His technique was to make geometrical patterns on his bed sheet, using lentil shells he'd saved from his meals.
By the end of the film, the audience is allowed to see the sentence to which Turk refers at the start of the film, which might be phrased as a question or an exclamation, spelled out in lentil shells against a bed sheet.
It seems Turk's diet was comprised nearly entirely of lentils and rice and after some time he diverted himself by collecting the lentil shells
. When he'd collected enough of them, he used them to create geometrical patterns within the only frame he had at his disposal, the white sheet on which he slept.