There are a few different ways of inserting a zip. One way was described in the zip section of the fastenings page. This described the way to insert a zip before all the rest of the seams were sewn up.
Measure the zip 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 the rest of the seam along the seam allowance.
Open up the zip and place the edge of the seam along the edge of the teeth. Using zip foot on the machine sew in place.
Close the zip fastening. Spread the cover out flat,right sides up. Bring the other side of the cushion cover over the teeth to meet the sewn edge. Pin the zip in place on the right side making sure that the zipper tape is secure underneath.
Top stitch along top of zip. Keeping the machine needle in the fabric, raise the foot and turn the material (pivot) to sew down the other side of the zip using the pins as a guide.
Pivot again and sew across the end of the zip.
Open zip, turn the cover wrong side out and continue to sew the other sides of the cover. Turn right side out, place cushion pad inside and close zip.
The above version can also be used to insert a zip when the rest of the other seams have already been sewn up. This can be a little tricky but it really depends on which way suits you. I prefer to insert the zip first.