Pin the zipper to the center of one of the 10” sides of the leather. The top side of the zipper and the pretty side of the leather should be facing each other.
Begin sewing the zipper to the leather. Insert the needle to the wrong side of the leather, about 1/4” away from the end of the zipper (unzip it if it makes it easier to sew). Make a simple straight stitch along the zipper, close to the teeth. Don’t worry if your stitches aren’t perfect.
When you reach the end, knot the thread about 1/4” on the outside of the other end of the zipper. Insert the needle through a bit of the zipper fabric and pull thread until there is a small loop remaining; then insert the needle through that small loop and pull. This makes a firm knot; repeat a couple more times.