How to create gathering stitches
Working on the right side of the fabric, tack two rows of straight, long, parallel stitches in the seam allowance; one just inside the stitching line and the other 6mm away. Be sure to leave long thread tails. Secure one end of the thread tails so they don't pull out accidentally. Pull the loose bobbin thread tails to gather the fabric, sliding the fabric along the threads. Loosen the tension on the needle thread so the bobbin threads are easier to pull. When the gathered piece matches the length of the corresponding ungathered piece, pin the layers together, adjusting them so they are evenly distributed. Change to a shorter stitch length and sew the two layers together. If the fabric to be gathered is long, divide it in halves or quarters and tack and gather each section independently.