Anyone who has been knitting for a while has experienced how monumentally boring it can be to wind a ball of yarn by hand. Maybe your local yarn store is kind enough to wind those lovely, long, twisted skeins of yarn into something you can actually knit from. However, you may not always have time to wait for your yarn to wind. Some stores don't offer this service. Perhaps you even spin your own yarn. But if you have an electric drill and some odds and ends, you can wind your own yarn balls whenever you like.
Cut a small X into the center of the plastic container lid with a knife. Push the stem of the beater through the X so that the lid is flush with the beater. Super-glue the lid in place and allow to dry. The lid will prevent yarn from slipping down and winding around the stem of the beater.
Insert the beater into a toilet paper roll. It will be a tight squeeze, but it should fit. Cut a 1/2-inch slit in the outer edge of the roll.
Insert the stem of the beater into the electric drill and tighten the drill around it. Before winding the yarn, you need to set it up so that the skein will unwind without tangling. You won't be able to hold and wind the yarn at the same time, so you will need a helper or a yarn swift.
Slip the free end of the yarn into the slit you cut in the toilet paper roll to hold it in place. String the yarn down to the base of the toilet paper roll, near the container lid, and wrap it around the roll a few times.
Set the drill at about the same height as the yarn swift or your helper's hands. Slowly squeeze the trigger to wind the yarn around the roll. As the roll rotates, use your free hand to guide the yarn up and down the roll so that the yarn builds up evenly across it.
Maintain enough tension to hold the yarn on the toilet paper roll but not so much that it is extremely tight. High tension will stretch out the yarn and create problems with gauge when you knit with it.
Remove the toilet paper roll from the center when the ball is finished for a center-pull yarn ball, or leave the ball on the roll for an outside-pull ball.
If you're making a center-pull ball, leave the ends of the paper roll exposed to make it easier to remove.
Depending on the yarn and your winding speed, you may want to wear a glove on the guiding hand to prevent friction burns.