So the main thing you need is a T-shirt, make shore that it's at least two sizes bigger than you are.
The first thing you need to do is to cut the neck line just to make it more opened but you can keep the original one if you like it better. Than cut the two sides of the t-shirt so it widens at the bottom.
Now cut the bottom of the T-shirt as it's shown in the picture. This will make the T-shirt longer in the back and shorter in the front.
As you cut the sides of the T-shirt the front and the back are now separated, I joined them with safety pins, but you can saw them together or join them in a different way. DON'T FORGET TO LEAVE HOLES FOR THE ARMS.