The idea is that, once these pieces are adjusted to fit, the pieces themselves become the pattern pieces that are used to cut out and mark the real fabric. While the muslin is a bit flimsy, it's a lot sturdier than the tissue paper that the pattern is printed on!
By comparing the markings on the original pattern pieces, I was able to determine that each size was drawn 1/2 inch larger than the previous size. In order to adjust for the different sizes that I thought I'd need to fit Susan, I marked the smallest size at the bust, one size up at the waist and a third size at the hip, and then connected those marks with a french curve. Finally, I marked 5/8th of an inch inward from those lines, to show the seam allowance.
I did get tricked once - one of the pattern pieces only adjusted in size along one seam - not all the way around... Once I discovered that, I worked more carefully and so was not tricked by the two pieces (center bodice and godet) that were the exact same size for all versions of the pattern.
I will say that basting (I used the longest setting on my machine) and using two different colors of contrasting thread (in my case: black to mark stitching lines and red to connect pieces) did make removing stitches much simpler! I was glad I had followed that bit of advice! :)
Finally, I did not follow the instructions for assembling the dress. Instead, I basted up the lining to create a "stand alone" dress that Susan could try on first. In addition, I basted up the folds in the front of the outer dress, but didn't connect the pieces of the outer dress. I figured I'd have her try on the lining first, and then, depending on how it fit, assemble the outer layer accordingly.