First, I cut out the main body pieces based on the size of the embroidery on the pillowcase. I made sure to have at least 1/2" clear on all sides of the design for the seam allowance. I have two pieces, one from the front and one from the back. They measured 24" wide by 16.5" tall. Sew these right sides together at the side seams with 1/2" seam allowance. Finish seams.
I used the left over scraps from the four sides of the pillowcase that were cut away from the main body pieces to create the remaining pattern pieces from the tutorial. The opening to my pillowcase had a cute crochet border, so I cut the chest band from this piece. I did not have enough scraps to bifold the piece as the tutorial indices, so my piece was 21.5" by 2". Sew this side seam with right sides together and finish the seam.
I was able to cut the four shoulder pieces from the remaining pillowcase scraps. Create the bias tape straps as indicated in the tutorial.
Ashley gives some good tips on gathering the main body fabric of the dress. I also like to stitch a double row of gathering stitches, one at 1/4" and one at 1/2". This seems to keep my gathers straight and provides a backup if a thread breaks during gathering. I also snip a small cut, less than 1/4" to mark the center of the body fabric instead of using the safety pins. You will need to hem the bottom of the main body piece since you will not have a selvage edge. I just sewed the main body piece to the chest band with right sides together and finished the seam since I did not have the bifold piece to tuck the body into.