I saw instructions for this on a sewing blog.It's a very easy T Shirt dress. Take two t shirts, one large, one small. Cut the arms of the large and sew around the top in a large stitch to gather it.
Then stitch it to the smaller t shirt with right sides together. You can make the bottom of the skirt more flouncy by using a man's XL size T-shirt and adjust its length to suit. If you would like a lower waistline use a longer t shirt. The size 20 wasn't that much larger so there wasn't too much of a flounce. Next time I'll look for a larger size.The small gathering was a little unflattering so I added lace to hide the seam. To tie it all in, I put lace around the collar and cuffs too.You could also add either flouncy or puffed sleeves by using the larger t shirt's sleeves and gathering at the top and bottom. Or pockets from the left over material.
You rip what you sew!