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).
This finished project will look so good everyone will want to eat it!