In threading, a second thread or yarn is threaded under and over a previously worked base stitch. Threaded stitches are worked on top of the fabric that is the needle does not pierce the fabric and are best accomplished with a blunt needle. These stitches thread in, out and around the base stitch and is used for decorative outlines and borders. In Threaded Backstitch a line of Backstitch is interlaced with one or two threads.
Work a foundation of Backstitch.
Bring the second thread up at 'a' slide the needle under the next stitch and continue the pattern by passing it under the next following stitch weaving from side to side.
For a double threaded variation bring another thread up at 'c' and weave in the same way filling the gaps.