Cut a slit about ¼ inch (7mm) long. The cut will self seal when you withdraw the funnel. Make the cut under the trademark so you will be able to find the cut when you get ready to start filling them. Done this way you can cut a bunch of balls and then fill them.

Cut the tip of a funnel at a 45° angle for easy insertion into the ball. Use two heaping tablespoons of clean, dry sand to give good weight and a uniform product. These balls have very little bounce and are not prone to roll.

Tennis players regularly throw away old balls. The sand can be obtained from various sources. Concrete and asphalt producers have suitable (small enough to go through the funnel) sand in huge quantities and have never charged me for a bucket of sand. You can dry the sand on the stove. Heat at about 200°F. (92° C). Place a small piece of paper on top of the sand - when dry the paper will quit curling.

A drop of glue will seal the hole. (I use "Shoe Goo") Alternately, you can cover the ball with a 9" round balloon (cut the neck off the balloon).