The options for the 'cord' part of your seam are endless. The easiest thing to do is just cut a strip of fabric that you like, but you can actually buy cording for seams if you like. I tend to prefer just working with fabric, but it's really up to you.
Unlike some other seams, to sew a corded seam you do not start with a plain seam. Instead, you have to create the seam with the cording already in place. To make your cording, cut a strip of fabric as long as the seam and a little bit wider than double the seam allowance. So if you're going to leave ½" seam allowances, the fabric should be just over 1" wide. How much wider depends on how prominent you want the cording in the finished product. I like a very discrete cording, so I'd cut a piece of fabric that is 1¼" wide. You can go wider, if you like.
Now that you have your fabric strip, you can start your seam. You'll need pins to keep everything in place, so make sure you have plenty on hand. The key to a corded seam is to line everything up and pin it all in place before you start. You'll be lining up the raw edges of the seam allowances of both your project and your fabric strip. The seams of your project go right sides together, but the fabric must go in between. Fold your fabric strip in half lengthwise, wrong sides together. This is important. You want the right sides of the fabric strip facing outward.
Now that you have your folded strip, you can place it in between the raw edges of the seam allowances of your project. Line everything up perfectly and pin in place. You don't want anything to slip. Stitch as you would a plain seam, ½" from the seam allowance. Go carefully and straight, making sure you're catching the cording while you sew.
This technique can be used to apply many types of decorative trim. You can apply cording, yes, but also lace or any trim that has enough of an edge to slip between the fabric of whatever project you're working on. The possibilities are endless.
