How to couch stitch
Secure the thread at the back of the work and bring the needle up to the surface at the start of the design. Place a thread or cord along the sewing line and take the needle over it and down through the fabric on the other side.
Continue making stitches over the cord to secure in place along the length.
TIP: when working with threads or cords that have an obvious twist to the ply, use diagonal stitches at the same angle so they will be concealed by the cord.