I'm sorry it's not a How-To projekt, but I was not a member when I re-made this shirt. But it is not that hard to do.
find a oversize fan t-shirt. Then cut of the seems in the neck, arms and the buttom of the shirt. Cut 2 strings of the buttom and save them for later.
You should measure around your stomach, the spot where you are thinnest. Then measure the shirt where it's thinnest and then draw lines so the shirt will fit in the sides and cut the shirt. When you cut the shirt then make small holes in the sides and use the 2 strings you saved to tie the shirt together in cross stings in the now open sides. Like the photo shows.
I chose to make it like a off shoulder T-Shirt you can make it the way you want xD.