This is the ready-made back that turns a finished piece of embroidery into a cushion: the fabric is black cotton etamine and the zipper is already sewn in, so you simply place the back against the stitched front, right sides together, sew around the four sides and turn it out.
The zip makes it easy to take the pad out when the cover needs washing. The embroidered front and the inner pad are not included: the back is made for Vervaco cushion fronts, but it works with any embroidery of a suitable size, on Aida or linen too, as long as you leave enough fabric for the seam allowance.