I think we've all seen that fun Knotted Headband by You Seriously Made That - - here's a refresher:
If you're wondering how to make t-shirt yarn, just cut strips from an old tshirt (mine were maybe 3/4 in wide) and then pull on them til they turn in on themselves. Sometimes the tshirts won't roll in on themselves - I've had this happen a couple of times. It just depends on the type of knit fabric they're made of - but most should roll in to form a "yarn". I cut 4 strips and sewed them together so I had 2 extra long separate strips. That easy.
Want to see my video tutorial on how to make this knot?? Check that out here: Knotted Headband Video Tutorial