Here the invisible zipper is already attached to the left back. I've pinned the other half of the zipper to the left front, marking the top of the pocket. Stitch the zipper to the front, stopping at the mark (top of the pocket).
(Side note: You can see I attach my pocket bags at the waist seam. Later you'll see I close the pocket below this. That way when I top stitch my waist seam, I'll pin the top of the pocket to the seam, and the top stitching will hold the pocket bag to the front. This helps me get my hands in my pockets and keeps the pocket from gaping open.)