There are a few different ways of inserting a zip and there is a way that each person will prefer.
This stage comes after the rest of the three sides of the cushion has already been sewn with right sides together and the wrong side of the fabric facing out.
Place and measure the zip length along the opening edge as before and mark start and finish of teeth with pins.
Stitch the seam to the start of the teeth, back stitch to secure the thread and then do the same at the end of the teeth. Press along seam using sewn ends as a guide.
Turn the cover right sides out.
Open up the zip, pin and place the edge of the seam along the edge of the teeth. If the zip is to be concealed then make sure to cover the teeth with the cover edge.
Repeat for the other side.
Using the zipper foot on the machine sew in place keeping equal distance from the edge.
All zipper feet are different and on the ones above the wider part of the foot is placed on the edge and the stitch is then just beneath the teeth
When sewing a zip in after the cover has been made it is important that the rest of the cover is held out of the way. Carefully sew down one side and in this case the zip cannot be closed so the stitch line may go askew when nearing the zip mechanism.
It is difficult to stitch across the bottom of the zip but it can be done with care. Then stitch up the second side. If the seams are sewn one at a time there may be some gaps at either end and this can be hand stitched closed.
I do prefer to put the zip in first and then sew the rest of the cover afterwards as the results are neater and it isn't so fiddly.
If a zip breaks in an existing cushion cover then unless the rest of the seams are unpicked this is the way it will have to be replaced.
Another alternative is to hand sew the zip in place.
Many people are put off by sewing zipper but keep practising inserting a zip and after doing a few it will seem easier.