I used a large shirt for this project. I cut the sleeves off to give me a large rectangle.

However, you can measure the length you want the dress and make the width a few inches bigger than your actual width.

Sew sides together.

Hem the bottom of the dress.

Make a case for your elastic to go through.
Fold the top down once and then one more time (make sure the fold is the correct width of the elastic)
NOTE: Make sure you don't sew the case all the way leave a little opening so you can thread the elastic through.

Attach a safety pin to one end of the elastic and thread through the casing you made.
When thread all the way through sew a box and a X in the box to secure the elastic together.
Then sew the case close.

Measure from the top of your bust to below your bust and mark all around the dress.
Take a second piece of elastic and sew all around the dress. Make sure while you're sewing you pull the elastic as you go.