If you don't know, dryer sheets and fabric softener are like the worst thing you can do to your clothes. Throwing balls in your dryer is a much better alternative without the chemicals. I've been using tennis balls in my dryer for awhile now (which still has rubber and whatever else they are made out of) and decided that I should upgrade myself to wool dryer balls. They are natural and will help dry your clothes faster, by moving the clothes around better as well as absorbing some of the moisture out of your clothes and have then be static free.
I wasn't sure what to buy for yarn, so I bought a couple to start and ended up going back for more. The yarn on the left is 100% wool while the yarn on the right was part wool and part something else (I don't seem to have the package still). They both worked just fine. The most important thing you are looking for is yarn that specifically says that it must be hand washed and cannot be machine washed. Both of these yarns said this. I also made more balls out of a gray and white yarn which is also the same as the pink one pictures above.
All you have to do is wind the yarn up into tight balls. When you get it to the size you want, you just use a crochet hook or something else to loop the end underneath other layers of yarn to keep it together. I made my first ones quite large, so I ended up making these out of 2 more sets of yarn. I used half a skein of yarn which seems to be a good size. It's important to have a few balls in the dryer so they work together to move your clothes around and the more wool you have in there, the more it will aid in drying your clothes faster.
Then you put them in an old panty hose, or since I don't have any of those a large sock will do. Make sure to tie off between the balls so they don't felt together. On my last set of balls, I simply put 1 in each sock and that worked also, just as long as your ball of yarn is nice and tightly put together. Now is the fun part. Wash them. Over and over again. Hot wash water, cold rinse water. I just left mine in the washer all day on laundry day and then put in the dryer with the last load. My second set, I just washed on a extra hot load and then put them into the dryer and that worked well for them.
Here they are, the finished product. They will continue to felt more and more as they are in the dryer. The most important thing is to just make sure that they are felting together and are hard to peel apart before you start using them. I'm very happy with how they turned out and that I can now my tennis balls can officially become kids toys!
I'm adding this comment a month after the post. I like the grey and white balls the most with how they have continued to felt and look, but that is really just a personal preference. Also, It seems that the larger the ball, the better. The little grey ones more often than not get stuck inside the sleeve of a shirt or somewhere. I'm always taking them back to the dryer. Whereas, the pink and white ones normally reside in the dryer. If I ever make more, I would make 1 ball out of 1 skein of yarn. I have found that my clothes have more static with these than with the tennis balls I was using, but my clothes are certainly getting dryer faster with the wool balls. The dryer is always done 10 or more minutes before the washer. I'm going to stick with the wool balls over the tennis balls!