Step 4: Make the cushion back
Measure your cushion front - it should be about 22 1/2" square, but may have contracted a bit with the quilting, mine measured up at 22". You now need TWO pieces of fabric for the cushion back, the length each will be the length of the front and the width will be 2/3 of the width, so in this case, 15".
On the wrong side of the fabric draw a line 1" in from one long edge on each piece. Fold the fabric to the line and press along the fold, then fold over again at the line and press again to enclose the raw edge. Sew along the fold to secure the fabric in a 1/4" hem. Repeat for the other piece of cushion back fabric. I chose to use the green thread for this because it looked more attractive and was easier to see for the photos.
Lay the cushion front right side down and then place the two cushion back pieces so that the hemmed edges overlap each other and the raw edges of the back fabric match the raw edges of the cushion front. Sew all around the raw edges as close to the edge as you can, this line of stitching will be hidden by the binding, so again I used a contrasting colour to make the pictures clearer.