A fitting muslin is simply the creation of a pattern using muslin or other inexpensive fabric.  Creating a muslin allows you to test drive a pattern to discover any problems with construction or fit, and to make major pattern alterations more easily than with tissue (easier for me, anyway).

I made up the edited McCall’s 6819 jacket first, without the skirts.  (I made the duct tape dress form of my daughter about a year ago, so I have a lot of confidence in sizing my patterns for her.)  I can see that the sides can be taken in about an inch in order to make it fit more closely than the pattern calls for, and that the shoulders should be taken in about 5/8 inch.

Next, I drafted up a hood pattern to mimic the character’s and gave it enough length so that it would roll under (per the illustration).  As it turned out, the hood was deeper than we liked.  In the photo below, we’ve turned back the edge about 1 1/2 inches.

As expected, the shoulders and sides came in a bit.  The peplum was right where she wanted it.  I measured the length for the skirts at front and back.  The angle of the peplum would determine the rise from the center front/back points to sides.

The next step was to alter the skirt pattern to create the shorter, angled skirt.  Here’s the Ezio illustration as compared to the McCalls pattern:

To do this, I laid out the front skirt pattern and measured the length.  Keeping in mind that this length includes seam allowances for the waist and hem, I subtracted the desired skirt length, which gave me the amount by which I needed to shorten the pattern.  Using the indicated line on the pattern for lengthening and shortening adjustments, I folded the pattern down to subtract the necessary length.

Next, I laid the pattern piece for the front center peplum piece and aligned it with the front center edge of the skirt.  Using the bottom of the peplum piece as a guide, I lined up my ruler and marked the angle across the bottom of the skirt.

I now had a skirt pattern that would rise to the side at the same angle as the peplum.