Zip Panel
You don’t have to add a zip. There are easier options, for example you could use Velcro. You could also sew the back panel by hand up once the cushion insert is in, but this means you can’t remove the cover to wash.
The zip panel wasn’t too tricky. You need to cut two strips of fabric (as opposed to one like we did for the front panel). The seam allowance is required around each of the two pieces. So, if your strip needs to be 8cm wide, you will need two strips of 8cm each. That’s 8cm divided by 2 = 4cm + 2cm seam allowance all around = 8cm.
I found it easiest to tack the two panels together using a contrasting thread (makes it easier to identify and remove later). Then fold out the seams and tack the zip into place.
I’ve used big stitches to hold my two pieces of fabric together (outside facing in) along the seam line. Now you can see the seams of the two panels have been folded over (you could iron them flat) and then the zip tacked in place. Make sure the zip is facing the right way for use.
Using the zipper foot on the sewing machine, sew your zip into place and remove the thread for tacking.
For a tidy finish, add a small panel of fabric to each end of the zip, before attaching piping, as illustrated below. Just fold the end of the fabric over and sew all the way down the panel end (including over the zip).