Made from two XL t-shirts. I would suggest using a size one to two sizes larger than you normally wear for easier alterations.
1) Cut the neckline and sleeves off the first shirt. You should be left with a sort of tube that's all floppy and loose at the top. Pin the top together so that it rests above your bust.
2) Put the shirt on your dress form or on yourself. Start pinning up the sides until you reach the top, following the natural curves of your body.
3) Once it's pinned to fit your shape, carefully remove the dress.
4) Start sewing along the pin line, making sure not to deviate from the shape or stitch over the pins.
5) Try on once sewing is complete. Make any adjustments necessary. You should now have a fitted tube dress.
6) Take the sleeves you previously removed and cut the seams off until they lay flat. Put the tube back on yourself or the dress form and pin the sleeves with the finished edge facing inward.
7) Take off the dress.
8) Take the second shirt and remove the neckline, sleeves, and bottom hem. You should have a giant rectangle at this point. Cut the rectangle in half lengthwise, and then take each piece and cut in half again. Sew together at the short sides until you have one ridiculously long rectangle. Sew the last two open sides together and form a giant loop.
9) Pin the loop to the side seams of the dress. Start pleating the remainder of the loop in both front in back.
10) Once pleated and pinned, sew it down. You now have the bottom ruffle.