I have been using dryer balls for years now. I have tried my hand at making my own and even used pre made ones. My favorite so far are Vita's Wool dryer Balls from Amazon (because I swear everything I own comes from Amazon, oh the joys of living in a small town.) They are about the size of a soft ball.
What does a dryer ball do? It cuts down your drying time and keeps static cling down. With out having to use chemicals. Did you know if you have a dryer with an electronic sensor you are not supposed to use dryer sheets? Two different repair men have told me this, they explained that it builds up on the sensor and burns it out. So if your dryer has a sensor, which most current dryers do, ditch the dryer sheets and pick up some wool dryer balls.
So how do they work, as they bounce around your dryer they add more space between your clothes letting the hot air reach more area, the wool pulls in moisture, as they do this it cuts the static cling and keeps your clothing from getting wrinkles. You can also line dry stuff, pull it in when it is still a touch damp and toss in the dryer with the balls for just a couple minutes and it softens it up. So you get the best of both worlds, saving energy and that line dried smell, but the softness of drying in the dryer.
How are they different from the plastic ones or just using tennis balls, for one you don't get the moisture absorption so your drying time is no different, and you are heading up a plastic that could be releasing harmful chemicals into your clothing. So how do you use them, just toss them in and dry as normal. A normal load takes 4 balls, I like to use 6 for my loads of towels.
After you use your balls for a period of time they can get pilling, like on a sweater. This is nothing to worry about, they will work the same. If it really bugs you, you can use a sweater shaver and shave the surface of the ball. B
If you really miss the smell of dryer sheets you can add a few drops of an essential oil onto the ball and dry as normal. I love doing this because I can change up the scent depending on my mood.