First get a long sleeve t-shirt. If the sleeves have like cuffs on them then go ahead and cut them off.
Now focus on the sleeve. Start a couple inches away from the seam of the shoulder, and start cutting slits. Make sure the slits don't go any further than half way into the sleeve.
After you cut all the way down the sleeve, you should have something that looks like this.
Now stretch each individual piece.
This is the tricky part. I actually found this part easier to do while wearing the shirt. Ok take the piece at the very top, now pull the second piece through the first piece. Now pull the third piece through the second piece and so on.
When you get to the bottom cut the very last piece in half, then tie it around the previous loop you made so that it's secure.
Now you can do steps 5 and 6 again to the same sleeve which will give you this.
Now just go and do that to the other sleeve. :3