There are a number of ways to finish the table runner. I experimented with a new idea and was happy with the results. The steps below explain it.
Lay the fabric for the sides—we’ll call it Fabric A—on your cutting table right side up. Place the batting with the string path on top of it, right side up. This may seem counter-intuitive, but trust me on this! Trim Fabric A to match the edges of the batting and pin in several places to keep the pieces aligned.
Set your machine on the longest straight stitch it will sew. Stitch through all layers along the lines you drew in the very first step. These lines are covered with fabric now, so you’ll have to estimate where the lines are, based on where you trimmed the string path. It isn’t critical that you sew exactly on the original lines.
Remove the long stitches, and bring the Fabric A pieces around to the top side. Lay them in place beside the string path, with outer edges of the Fabric A pieces matching the edges of the batting. Fold under ½” along the curves and pin the edge in place on the string path. You’ll need to clip any concave curves almost to the seam line every ½” to get a nice, smooth fold.
Using the same thread that you plan to quilt with, topstitch as near the folded edges as possible, sewing through all layers.
Before cutting the back to fit, lay it on your cutting table right side up. Lay the table runner on it, wrong side up, and pin the edges. You may trim the backing fabric to match at this time or after you sew around the edge, which is my preference.