When decorating, always keep in mind that no-one's going to see the bottom/back/inside. This is only one small step up from wrapping a loose piece of cloth over a cushion, but it does keep the cover tidy and in place.
So: lay that piece of grey denim you're never going to make a skirt from, face up on the floor. Put the cushion more or less in the middle. (Please excuse the blurry photos, but it was the middle of the night.) Oh yes - zig-zag around all raw edges first.
Fold the corners up more or less like this.
Turn the cushion over to check that everything's neat.
Whip the 'cover' off.
Draw a straight line from the pinned corners to the edge.
(And then it was morning, and the light changed completely!)
Sew along the line, and cut off what you won't need. Probably a good idea to check that your seam's in the right place before cutting, though.
Put the cover back on the cushion.
Fold the corners, then fold the sides up. Adjust the folds so that the edges don't quite meet.
Mark button positions with pins...
... and sew buttons on.
Cut 4 lengths of thickish thread, about 20 - 30 cm long, double each piece up and knot the ends together. Put the loop around a button.
Wind the other end round and round both buttons, pulling them closer together.
Voila! Hey presto! Siedaar! etc. A zipless cover you can take off to wash, but don't have to tidy every morning.