If you want to make small balls of yarn quickly, a regular food fork works great!
Wrap the fork: Wrap the twine around the center (blade) of the fork. Wrap quite loosely at first so it's easy to get off.
Tie in the middle: Cut a separate thread of about 30 cm. Thread it between the center prongs (tines) of the fork and tie a tight knot around the wrapped yarn before pulling it off the fork.
Cut and shape: Pull the ball off the fork, cut the loops on the sides and trim to the round shape with sharp scissors. Boil a pot of water and put the balls in a colander over the steam to get more fluff and smoother balls.