Start by folding your fabric in half with selvage together. This will make the height of your fabric rectangles 22 inches. Cut the width at 20 inches and cut the fold to make 2 rectangles for each dress that are 20"x22". With the selvage at the bottom, you will only have to turn the hem up once since there isn't a raw edge. I always love fewer steps!
Mark your armholes 3" on top and 5" down on one side. Fold rectangles in half lengthwise and cut your armholes. Stitch sides closed. Hem bottom and make neckline casing with elastic just as you would if using a pillowcase. (pattern link in sidebar)
One tidbit about bias tape if you aren't familiar with it, I wasn't when I first started, one side is slightly shorter than the other on purpose, as you can see in the above picture. Apply the bias tape with the shorter side facing up when sewing as in the picture below and this will ensure that you catch the longer side on the inside of the dress more easily. I know, genius right? Who knew they did that for that very reason??? ;)