I looked at a lot of allegedly easy online instructions for cushion covers and they all made my head hurt, so eventually I decided just to sew a square of fabric to the back of the design. I did get all fancy at the end and add a zip.
One 69p IRMA cushion from IKEA and my work was done.
(I would like to point out that I don't own a sewing-machine, nor would I know how to use one if I did, so this is all literally my handiwork.)
callmemadam, I believe we had some sort of agreement whereby you now have to finish your tapestry cushion cover? 