Start with a blouse and two pillowcases.
Try on your blouse and determine where your waistline is. Make a cut 1" below your waistline to allow for the casing.
Cut your pillowcase seams apart along the side and the top. Determine how long you want your finished dress to be and add 1" extra for the casing. Cut away the excess. Lay your pillowcases right sides together and stitch up each side seam.
Your pillow cases are probably wider than the blouse. Run two rows of basting along the top edge of the pillowcases. One at 1" and the other at 1/4". Gather up the basting until it is the same width as the blouse.
Place the gathered pillowcases and blouse, right sides together, and stitch along the 1" basting line.
Trim away the excess blouse material from the casing to eliminate any unnecessary bulk.
Fold the gathered seam upwards towards the blouse and stitch in place along the 1/4" basting line. Be sure to leave an opening for the elastic to thread through.
Thread your elastic through. Adjust to the right size. Cut and secure the elastic. Stitch the opening closed.
At this point, your dress is done. I wanted to make the neckline more modest so I ruffled the extra blouse fabric and stitched it to each side of the neckline.
I also wanted a matching belt. Sorry, no photos (it was an afterthought!) I used two layers of heavyweight iron-on interfacing to give it shape. It is secured with velcro. The flower was made by ripping 1 1/2" strips of fabric, gathering them down the middle, then sewing them in a circular pattern onto a circular base of fabric. (I strengthened the base with two layers of interfacing also.) I started sewing around the outside edge and worked my way to the middle, keeping the rows very close together. I topped it off with vintage earring hot-glued to the middle.