Two easy ways
#1-This method is great for gathering large pieces, or heavy fabrics. You will need heavy thread, or kite string.
A- Set your zig zag stitch to its widest, and longest setting
B- Put your fabric under the presser foot set to sew at 1/2".
C- Set your thread on the fabric so that it will be in the center of the zig zag stitch.
D- Begin stitching over the thread. Make sure you don't sew through the thread.
E- When you are ready, pull the strings to gather the fabric easily.
#2-This method works best for smaller pieces, or light weight fabrics.
A-Set your straight stitch to its longest setting.
B- Starting at a seam, stitch 1/4" from the top edge of the fabric to be gathered. Don't backstitch at the begining, or the end.
C- Sew another row of gathering stitches 1/2" from edge.
D- When you are ready to gather the fabric, gently pill the top threads of upper, and lower line of gathering stitches.
E- Pull thread tails from both ends until the gathers are even.