I needed a white strapless dress for my Storm cosplay but couldnt find any that I liked or were my size so I decided to make one out of the 2 white t-shirts I got months ago.
First cut both T-shirts where the sleeves meet the body. (Green dotted line)
So now that you have 2 tubes, turn them inside out and pin the raw cut edges together to make one big long tube . Sew them the tubes together. I sewed about 1/2inch from the edge.
With the dress still inside out, trim the raw edges of the seams to about 1/4 inch. Use a wide zigazag and stitch over the seams. (see pic)
You should have one big tube. With it still inside out decide which end you want to be the top and which to be the bottom. Sew elastic, about 3/4 of an inch away from the hem or where the hem stitching is, all the way around the tube using the zigzag stitch. Dont forget to stretch the elastic as you sew.
Decide where you want the front to be.
Mark with pins where the front ends on each side.
Measure 7inches down from the elastic you sewed.(purple line)
Sew elastic across the front of the dress only.(yellow line)
Turn it rightside out and your simple dress is done. If you want you can do like i did and use the spare fabric from the sleeves and neck and cut and sew on ties for the dress.