Pivot method for flat shoulder blades
Turn the shirt inside out. Using the ripper to release the shoulder seams, shoulder darts, and back armscye seams. The "armscye" is the armhole, from the tip of the shoulder to the edge of the bust, under the arm and back around to the point you started, according to Sempstress.org. On the shoulder, measure 1/8 to 1/4 inch out from the shoulder seam and mark. Using the ruler, check that the new seam line is parallel to the old one.
Let out the darts equally until the blouse lays smooth, with no tension over your shoulder blades. Once you achieve the right fit, straighten and shorten the darts as needed. Mark, pin, and sew the darts using a straight stitch.
Draw new armscye seam lines to restore the length of the shoulder seams. Blend the new seam line into the original armscye. in the arm hinge area. Draw each new shoulder seam line across the now stitched shoulder darts. Pin and sew using a straight stitch.
If you find yourself having to do a sleeve alteration, consider consulting a seamstress.
Consider hand stitching the last 1/8 to 1/4 inch before the end of the seam for a smoother finish, those tiny areas may be hard to catch on a machine if you don't release the sleeve in the shoulder area for the adjustment.