I made these in no time at all - the trick is to use big knitting needles and nice chunky yarn.
Knit a rectangle that is long enough to go around your cushion and then a bit extra to overlap (about another three inches). Sew up the sides with extra yarn and your yarn needle to make a sort of envelope. Slot your cushion inside and add a button(s). I've not bothered making button holes, as the knit is chunky enough that you can just pop the button through.
The black version has a bright lime green contrast on the other side and has been done in a simple stocking stitch (knit one row, purl one row and repeat).
The blue version is in moss stich (knit one stitch, purl one stitch and repeat).