This is super easy (hence the 5 minute part of the title). All you need is an old t-shirt and some scissors.
You'll start by cutting about an inch into the bottom of the shirt. Start cutting the inch-wide strip off of the bottom, and going up in a spiral until you reach the sleeves. Once you hit the sleeves, you'll cut along the bottom seam and then make cuts in a zig-zag pattern like shown. Repeat on both sides.
After this, you'll have a square left around the collar. Cut around the outside until you have reached the collar. Then all you have to do is wind it up into a ball. Voila!