Flexible measuring tape
I made mine before I was doing my craft blog so I didn't take pictures of the process. Luckily my friend made one and she let me take pictures of hers while I was helping her with the zipper part.
Here are her two pieces of fabric already cut out (note: they're both still folded in this picture)
Unfold each and pin the "front" of the boppy cover (see picture below) with right sides together and measure and mark 12" out from the center in each direction. This is your zipper's length. Now mark 2" out from those marks on each side. Using a 1/2" seam allowance (note: the original pattern says to use 1/4" but I liked it tighter-fitting), sew a regular stitch starting at your outtermost mark and then switch to a basting stitch at the zipper's mark. Baste to the opposite side's zipper mark then regular stitch those last two inches. Sounds harder than it is. Refer to my throw pillow cover tute for the same instructions worded a bit differently :)
Once your basting stitch is done, iron the seam open.
Then pin your zipper, with the teeth directly on the seam, to the seam allowance only (not to the cover) and sew as close to the zipper teeth as you can using your zipper foot. You will stitch a straight line on each side of the zipper.