You'll need about three ounces of wool yarn or roving to make each dryer ball. Any type wool can be used to make dryer balls, as long at it is not labeled "superwash". Other animal fibers like mohair, alpaca and llama will work as well, though these are generally more expensive than wool. Acrylic, cotton and other man-made or plant based fibers will not work for dryer balls.
To make a wool dryer ball, wind the yarn around your fingers ten times to create a loop, then slip it off. Continue wrapping yarn around the bundle; the more times you wrap it, the larger the ball will be. Stop wrapping when the ball is the size of a softball. Insert the yarn ball into the leg of an old pair of pantyhose and wash in the washing machine. The ball will felt into a solid mass. Once felted, drop the ball into the dryer to use.
If you have scrap wool yarn leftover from other projects you can use it for your dryer balls. The only color that will show is the last one you used, so put colors you find unattractive or that you don't care for into the interior of the ball. Use needlefelting or embroidery techniques to decorate the outside of the ball if desired.
Create a set of three matching dryer balls to give as a gift or sell at a craft fair. Add a few drops of lavender or rosemary essential oil to the dryer balls before placing in the dryer to add a gentle and natural scent to your laundry. This technique can also be used to create play balls for cats or toddlers.