Trace your sloper (pattern with no seam allowance) and follow class/book instructions for pattern modification.
Once the new design lines are planned, slide the ruler between the original traced line and the new marks until there is a graceful blend between the two. (A) You may need to slide the ruler more than once, marking only a small section at a time, until your new line smoothly transitions from the old one.
Altering Flat Patterns (B)
Place marks on the pattern indicating the position of the new line.
Slide the ruler to blend the old with the new. (B)
Always mark from the stitching line and add seam allowance after the new line is complete.
Fit the garment.
On the wrong side of the fabric mark the pinned fit line.
Follow the pattern drafting instructions above to slide the ruler from the existing stitched seam line to fit marks, until you have a new gracefully curved line. (C)