Start a ladder stitch from the inside of the pillow by pulling the thread through the folded seam, right behind the opening that you created.

Make a small stitch through side of the opening.

Make an identical stitch directly across from your first stitch, on the opposite side of the opening. Pull the thread taut to begin the new seam.

Repeat, alternating from one side to the other until you have reached the end of the opening.