Perhaps you also have a small child who is prone to pressing the unnecessary lock button on a door, nd therefore locking themselves out because the only key to that room is somewhere in his room. You could replace the door-set with a regular passage set that doesn't have a locking mechanism, but the lock may be useful later, or perhaps you are living in a rental, or maybe you want to make the door know easier to grip for elderly, or to help increase torque for stiffer door knobs. This is also a nice way to recycle otherwise unrecycleable tennis balls, or to brighten up tired door knobs.
Step 1: Disable the Push Button, If You Have One
If you have a push- button lock, you should know that a tennis ball won't be sufficient to prevent the button from being pressed. This is a problem if you are going to also cover the side that takes a key, should you need to unlock it from the outside. Rummage around in your recycling for something to sit over it and prevent the button from going in. I found the plastic sipper pull from a drink bottle ideal for this, but you may need to look at other options if your door know is a different geometry. I also used a ring of Blue-Tak (a sticky mouldable, non-drying adhesive like plasticine, in case you know it by some other name), just to hold it in place while I put the tennis ball on.
Step 2: Cut the Tennis Ball to Fit, and Install It
Take a tennis ball, and with a strong bladed paring knife, support the ball firmly and carefully and slowly pierce the ball to make your first incision. With a slow, long sawing motion, cutting in the inward stroke (less ball fluff), cut a round role, perhaps 4cm in diameter, rotating the ball as you cut. At this point, it will still be a little too small to force over the door knob, so test and cut some relief slits. This will allow the ball to be forced over the largest part of the door knob; you should not see them once the ball is fully on.
To install the ball, I found it much easier to twist as I push, and held the other side of the door-handle firmly, essentially screwing it on. Trying to just push it on will likely lead to frustration.
I recommend ensuring that you don't cut the hole too small, as it doesn't look particularly tidy like that, and you will likely get more ball fluff. Similarly, if you needed a way to disable a locking button, then make sure it doesn't stick out too far, or it will distort the shape of the ball and you'll see the relief slits you made, spoiling the look.