Back Opening

I first serge all the straight seams:

Then I place the backs right-sides together to sew the central 5/8" seam. I place a pin where the pattern has indicated the end of the opening. I set my machine to baste and sew to a couple of stitches beyond the pin. Then I set my machine for a regular-length stitch, back-stitch a few stitches, then sew the rest of the seam:

I press this seam open, then gently pulling apart the seam holding the fabric up to a light, I can tell where the large stitches stop. I place a pin there. Now I will sew around the basted area, including area just beyond the pin:

Now I clip the basting stitches, pulling out the clipped threads afterwards.:

The finished opening:

The next step is to sew the back to the front only by the shoulder seams and then apply the binding to the neck and armhole edges. Here is the blog post about bias binding.