This square cushion is seamed on all sides, with an all-over slip st pattern that creates interlocking vertical lines by changing one colour every 20 rows. For this pattern you work two RS rows, then two WS rows, one using each colour, sliding the work back and forth on your needles to do so, so you must use circular needles not straight ones, even though you’re knitting flat, not in the round.
When measuring row tension, remember each row is effectively worked twice, once in each colour, so you will see 22 sts fit vertically into 10cm, equalling 44 rows of pattern.