I made one by adding the fabric to the hem of the t-shirt, rather than at waist level, that made it easier to do because the hem was already reinforced. The gathers made it sort of a dropped waist look and I only took the fabric to my knee. I used 1 yard, cut down the middle lengthwise, as I only needed about 18" of fabric drop.
I will add, my sister bought one of those long dresses at Walmart last summer and loved it so much she went back and bought 3 more! She says they are the most comfortable things she owns. She got some of the ones with sleeves also and wears them almost everywhere. They wash and dry very well also.