Since I'm running a cushion swap over on Instagram, I thought I would share a quick tutorial on how I like to make a nice neat zipped cushion back.
Start with a zip longer than your cushion width. Cut a piece of fabric approximately 2" high by the width of the cushion for the bottom of the back. Lay your zipper teeth down onto the right side of the fabric strip and pin in place, so the zipper tape lines up with the top edge of the fabric. stitch along the edge using your zipper foot on your machine to help. Fold the fabric back, press and top-stitch along the edge of the zip. For the larger top section, cut your fabric to size across the width, but add extra (approximately 2")to the height to cover the zip. Lay the zip teeth down onto the right side of your other cushion back, so the tape aligns with the fabric edge. Pin and sew.
Now at each end of the zip, you are going to sew an 'L' shape to stop the zipper and create the concealed fold. With your fabrics still right sides together and the zipper in the centre with the pull in the middle, mark out up to 2" in on the width along the zip and up to 1" down (or the width of the zip tape). Do this at both ends of the zip, and then sew along your marks, ensuring you run across the zipper teeth a few times to make it extra secure. Now fold back your fabric and press carefully over the zip. Next, top stitch along the other side of the zip through the fabric flap, lining up along the zipper teeth to hold the layers in place. Then, trim off the tape in line with the fabric edge and snip down either side of the tape carefully to avoid the fabric. then cut across to remove the teeth from the L shape sections to reduce bulk on the seams. Make sure your cushion back is the right size and square.
To join the back to the front, you first need to decide if you are binding the cushion or not. Above shows a cushion ready to be bound, with the front and back pieces simply aligned wrong sides together and a stitched line around the edge, less than a 1/4" from the edge to hold the layers in place. You can then trim the sides neatly and sew on the binding of choice to finish, covering the joining stitching around the edge. If you don't want to add binding, place your front and back right sides together, with the zipper open and stitch all the way around using a 1/4" seam allowance. Turn the cushion through the gap in the open zipper and press. For a nice alternative to binding or piping, you can topstitch around the edges around 1/4" in on the turned through and pressed cushion.