Make sure to start and stop at the 2 1/2″ mark. Now, pin the other side of the zipper to your other square of fabric.
This next step can seem tricky. You are going to put on your zipper foot and sew a seam that will close up that 2 1/2″ space on both sides of the zipper. Below I have shown exactly where your seam should be and where it should start. Notice it starts about 1/2 inch from where the first seam ends. It is ok that the seams don’t meet up. When you press the pieces, it will lay really flat. In fact, the closer you get to the first seam, the more pucker-y it will be.