Find a shirt that you don't actually use. There's no specific fabric needed, any type will do. For me, I found this old goofy shirt that I bought last 2 years ago. And then just simply follow the pictures here:
Cut the sleeves. It's up to you if you want a bigger armhole. Don't mind it if your cut is unclean it really don't matter because after you cut the sleeves just stretch it and you're going to have a clean look of your shirt.
Let's get to the back of your shirt. Make sure you're going to have an even cut, with the use of tape measure measure the width of your shirt.
Fold it in half with an equal length in both sides. You decide how long your cut will be.
Gelato Slippers / Forever21 Denim Jeans & Camis / Avon bangles and necklace / Old shirt