A variation off of a project from the Generation T series, I made it from a larger t-shirt and added a belt to give the dress some shape. Will make Tutorial upon request
Start out by turning the t-shirt inside out and laying it out on a flat surface
Cut off the sleeves and make a straight cut across the top through both layers below the neckline.
Fold the top edge down of one layer an inch and pin down. Sew across and repeat on the other side.
Turn the shirt right side out and thread a drawstring fabric through both casings and tight the ends.
Add a waist belt and you're set to go. Make the collar as tight or as loose as you want.