This sure beats the "old fashioned way" of ruffling! Super tutorial for Ruffling and Gathering on your sewing machine with no threads to pull and no special foot needed. A winner! set your sewing machine to longest stitch with the tightest tension.
To get the right armhole, fit the bodice. Sarah Veblen in Threads. Learn how to successfully fit an armhole by using a muslin; Adjust bust, back, and shoulder—and your armhole almost takes care of itself.