Updated: Nov 12, 2020
After cutting the t shirt apart what you have left now is the body of the shirt without the hem and the sleeves. You have a tube which is open top and bottom and closed on both sides. From the one side to the other fold the shirt but stop about 6 cm before it meets the other closed side
Cut strips from side to side, crossing the fold but NOT cutting the other closed side of the shirt, stop about 2 cm before the edge.
You now should have a tube of fabric which has a closed section and strips which are attached to this closed section. Place a piece of cardboard inside the “shirt” turning it so that the closed uncut section is facing you. You are going to cut it now.
If you cut from the first cutting line through the closed section to the next cutting line you will just get many loops. You don't want that. What you want is one long joined spiral. To get that you cut from the first cutting line to the SECOND cutting line opposite that. All the next cuts are the ones facing each other. This way you will get one long spiral.
Take the spiral off the cardboard
Roll it up into a ball pulling it slightly as you do and you will see that if you have used a stretchy t shirt the yarn will curl up nicely.
When you come to the end cut open that last strip and there you have it, a ball of t shirt yarn