I spent yesterday filling several tennis balls with sand, and a week or two before, I filled several differently sized tennis balls with sand. I'm happy to be able to help some others out.

1) acquire sand: I got my sand from a local beach. It was a bit damp, so I cooked it at like 350f for several hours. This also would have killed any bugs/bacteria etc inside.

2) cut open ball: Tennis balls can be tough, I found the most effective way to slice was to use a razor, like the kind of point razor for making models. The cut itself I made directly into the felt surface (ie not the seams). I found, that for re-sealing purposes, two incisions forming a "V" work well, where each line of the "V"is about 1 cm.

3) straw device: to get the sand in, I made a straw device. Basically, I funneled the sand through a straw to get it all in the ball. Problem was that just a plastic straw gets pinched shut after being squeezed through the very smally tennis ball incision. Solution was to hacksaw an old ball point pen cartridge, resulting in a hollow tube about 2 inches. This tube is jammed through the ball hole, and the straw put through that. Perfect.

4) putting in sand: I worked with the sand and balls, and everything in a big basin, to preven mess. Handfull by handfull I'd just pour sand down the straw. As the ball fills up, the sand gets harder and harder to fit in. Tapping and shaking the ball to adjust the sand inside will be necessary.
I kept a small scale handy to moniter the weight, I managed to keep the difference between the heaviest and lightest ball only 0.3 oz. Cool

5) seal balls: Since I haven't put my balls through much use yet, I may end up altering this part of the instructions. I usd elmers school glue, and spread it along the inside of the incision, letting it settle in, and putting more and more in. making sure that it was well worked in and touching each surface of the cut. After that I just let them sit. As hard as It was, I waited until the following day before throwing any of them.

Good Luck with your ball construction.