Step 1. Draw line A to B as illustrated. This line should be at a level where the most rounding or protruding fullness is on your back. Then draw line C to D as shown. This line does not have to be at a right angle to A-B, but the angle that you choose will affect the final position of the dart therefore, it should be at a pleasing angle for your specific body shape. You may need to sew a couple of tests to make sure you’re pleased with the results.
Step 2. Cut from A to B and from C to D, leaving a paper hinge at the armscye and at point D on line A-B.
Step 3. Spread the shoulder area of the pattern open as shown always maintaining the paper hinge points. This will open up a small dart shape in the shoulder seam line and will automatically lengthen CB. Often people with rounded, protruding shoulder blades also have some rounding at the upper back and this extra length will help the bodice/top to sit more comfortably. Additionally, depending on how rounded your entire back is, you may also want to add a CB seam which is shaped/curved to suit your body contours. For Rounded Back instructions, see Page 14 #3 in the Minor Tune-Ups section of the Dress Kit Instruction Book.
Step 4. This technique for adding a shoulder dart shouldn't distort the armscye significantly. However, if the armscye appears too angled at the armscye hinge pivot point, make sure to blend/true to create a smooth curve.