A garment may fit everywhere, but if vertical wrinkles form along the outer edges of the back shoulders the garment is just too wide in this section of the pattern. This adjustment narrows the back, particularly at the back of the armscye, but the hem circumference and shoulder seam remains the same.
1. Draw a vertical line parallel to center back from the hem but stop at a point level to the underarm corner. Angle the line from this point to the outer third of the shoulder seam.
2. Draw a horizontal line from the underarm corner to the angle in the line from step 1.
3. Slash along both lines, leaving 1/8-inch hinges at the shoulder and underarm corner stitching lines. The vertical line is slashed through the hem.
Place a piece of tissue paper under the horizontal line. Push the underarm corner toward center back, so the vertical edges from step 1 overlap by the amount needed. The edges from the horizontal slash will open. Tape all slashes into place.
4. Close view of the horizontal opening slash at the underarm.
5. True the hem line. Depending on the original pattern design, the center back might need to be 1/4-inch to 3/8-inch longer. Make sure the side seam does not get longer. The front and back of the garment at the side seam needs to match
6. Finally, true the shoulder seam and you're done