Step 10: Sew in the Zip
So you are going to in a long rectangle around the zip.
Start in an easy place, like the middle
Sew down to the end and sew across to the other side
Sew down that side, across to the first side and to the middle.
Take your time and make sure you are getting through the outside, the folded under part, and the zip. It's easy to stop, unpick and go again so keep checking where you have sewn regularly.