Another method: loosen your upper tension or your lower tension [the thread you loosen is the thread you pull up] ; sew about 6 rows about ½” apart leave long thread tails pull up and gather up to desired width, steam lightly. Go back and stitch between the rows with a loose open emb design stitch, pull your basting thread out. This is mock smocking.
Don’t forget to set your tension back to normal.