Like so many decorative seams, you're going to start with a plain seam. Press the seam open. Make sure the plain seam is perfectly flat. The finished product will look better the smoother the original seam is. When it's as flat as you can make it, topstitch on each side of the seamline about ⅛" from the plain seam. Make sure the distance from the plain seam is the same on both sides.
You can venture further from the seam, if you like. You can even go up to ½" away from the plain seam. But be careful and make sure you catch the seam allowances while you stitch. Don't miss them. If you do miss them, you'll have to unpick the entire topstitch line and start again. It's a decorative seam, after all. It's designed to look nice.
This seam is simple and easy and makes a pretty little addition to a variety of projects.