If you're using a stitch pattern other than Garter Stitch, you may find it easier to work the increase and the decrease on the same row, as I did with my Bias Knit Scarf, which is worked in Stockinette Stitch.
Size One size (each cushion measures 18 inches by 18 inches) Gauge 6 stitches and 8 rows = 4 inches / 10 cm in stockinette stitch using size 9 needles.