I love how this turned out. I think the key to it is the multi-coloured yarn which gives it texture. It made it more interesting otherwise the cushion would be just plain striped.
I used a size 10 needle which is pretty large. It made the piece quicker to finish but not so easy on the hands. I cast on 65 stitches but it really depends on how large you want to make the cushion. I kept knitting in stocking stitch until I had a square then I cast off. The front and the back are alike so I knitted 2 pieces. All that remained was to sew up three sides, insert a cushion and sewed the fourth side up (so this cushion is permanently enclosed and I wouldn't be able to take it off).
If you look at the stitches, it is fairly loose. If I made another one, I would either use thicker yarns or knit 2 strands at the same time - just to make it thicker and less gaps.