Now we're going to sew along this line and then unpick part of it after we've put the zip in. Doing it this way ensures that the fabric closes over the zipper really neatly.
Using your normal stitch length sew from the edge to the first mark 8cm along.
Stitch backwards for 3 or 4 stitches and then forwards to the mark again.
Change your stitch length to the longest you have and continue sewing to the next mark 8cm from the other edge.
Change your stitch length back to your usual setting and sew forwards 3 or 4 stitches then back to the marker and then forwards again to the end of the fabric.
Press the seam open. The better you press it at this point the better it'll look when it's finished.