Here's an overall map of the bodice pattern changes.
Here's how I did it:
1. Underarm. The pattern, as published, has gussets under the arms for a close fit. They look like this when sewn.
I eliminated the gussets to form a dolman sleeve, which is similar to a kimono sleeve. This is a great option if you're feeling a little intimidated by sewing gussets or if you're using a fray-prone or delicate fabric (these fabrics may tear at the stress points of the gussets during wear).
It's helpful to use a French Curve ruler for this. Here's what mine looks like. (You can most likely find a household item to substitute, like a saucer or other small round object.)
2. Trace this underarm curve onto your back bodice pattern piece, so that the underarm seams match up perfectly.
3. When sewing, eliminate the gussets. Stitch the front bodice to the back bodice at the overarm seam, and then stitch the underarm seam. Reinforce at the curviest part of the underarm seam by stitching again 1/8" away from your original seam line (in the seam allowance) and then trim close to the second line of stitching. Clip and then press the underarm seam open above and below the underarm curve.
4. Waistline pattern changes. Draw a new seamline at the waistline curve on the dress front, side front, and back pattern pieces. Give yourself a 1" seam allowance (indicated by the dotted purple line) so that you can adjust the waistline if needed during fitting. Cut off the pattern below the 1" seam allowance.
5. You can add any skirt you like to the new bodice. Check out pages 98-103 in my book for full directions on making dirndl and circle skirt patterns. What I did for the Pinkie Pie dress was essentially a dirndl skirt, but with side pleats rather than gathers. Baste the new skirt to the dress bodice and then try on to make sure you're happy with the waistline placement.
That's all there is to it! Let me know if you have any questions.