Replace the sewn pieces on your workspace in the same position. Flip the backing section on top of the zipper section, and sew to the out facing only just as you did with the first piece.
It should look similar to this in the front...
...and this in the back with the inside facings both free.
Press under the seam allowance on your inside facings, and stitch down using a zipper foot. Adjust the zipper as you go to avoid hitting the metal pull with your zipper foot. I try to stay to one side or the other of my previous stitching by at least 1/16-1/8" or more, but you may choose to stay right on the same stitching line.
Time to square up. Using a long ruler and a rotary cutter, square up each side of your slab. Your rotary cutter should cut right through your excess nylon zipper length with no issue. Double check that the stitching you made across the zipper when attaching your 1" x 2"+ scraps does not get cut off.
I use my serger to finish my raw edges, but a closer zigzag on your home machine will work equally as well. Go around all 4 sides.
With right sides together, check the squareness of both pieces. These 2 did not line up well enough. I trimmed the lower edge, re-serged, and tested again.