Step 1; create a mock up, this will tell the tale of where and how much needs to be adjusted.
Step 2; have some one pin the excess fabric in the form of a horizontal dart at the waistline from the centre back being the widest point tapering to the side seam, until all the excess is gone. Take a look at the fit and see if it falls nicely over the butt (for lack of a better term!). When satisfied with the hang of the mock up, remove the dress leaving the horizontal dart in place.
Step 3; measure the dart at the centre back, it can be anywhere from 2 cm to 5 or 6 cm depending on the wearer.
Step 4; on the paper pattern on either side of the waistline at the centre back, measure the same amount taken in on the mock up and draw lines ending at the side seam.
Step 5; fold the paper using the lines previously drawn as a guide and tape or pin in place. Re-draw the centre back line so that it falls into a nice soft curve. The pattern will look a little odd but trust me on this, it will work! So before you continue, just to be sure..... create another back and using the same front, stitch together another mock up and try it on. You will be surprised at the results!
To my students, I will have a sample of this adjustment in class this week, so stay tuned.....
have a great day!