Hold the tennis ball in the palm of your non-dominant hand.
Cut half the tennis ball carefully in a large "X" formation with the utility knife. Don't cut beyond the middle half of the ball. Utility knives are quite sharp, therefore an adult must cut the tennis ball in half.
Ask an assistant to lift the chair leg while you spread open the "X" shape.
Slip the tennis ball onto the chair leg and release the "X"-shaped area. This allows the tennis ball to adhere snugly to any size chair leg.
Instruct the assistant to lower the chair leg to the floor. Repeat the steps with other tennis balls until you've covered all the chair legs.