Before I go too far, the rubber balls I used in my childhood and teen years were essentially naked tennis balls. One brand was made by Spaulding, which in our Brooklyn accents was pronounced 'Spalldeen' The spaldeen had a textured rubber surface and tickled nicely. The other brand was called a Pensy Pinkie and had a smooth rubber surface. It was good for teasing. Today you could use racquetballs.
The final way I masturbated with rubber balls was to use an improvised harness. I created a loop around my waist by 'daisy chaining' a bunch of rubber bands. You will need two balls for this technique. Getting the right sized rubber bands is also important. Those about a quarter inch wide and about four inches long work best.
It is topographically impossible to daisy chain the last two rubber bands in the loop together. So pull the last rubber band or two through the end rubber band so it makes sort of a lasso. Let the tail of the lasso hang loose for the moment.
Next, daisy chain three rubber bands together. Wrap each end rubber band around the ball so it forms an 'X.'
Take the free end of the lasso tail and loop the end over one of the balls so it is looped around the rubber band connecting the two balls.
Put the head of your penis through the rubber band holding the two balls together, and finish it off with a rubber band or two around the penis and tail to hold it in place.
Be careful with the rubber bands. You want them tight enough to hold everything in place, but not so tight as to pinch the skin.
Then get down on all fours and hump away. The balls swing back and forth, and jerk the penis while occasionally rubbing themselves against the penis. For a teenager, this may be enough stimulation for an orgasm. (As a teenager, I accomplished it a couple of times). For older folks, it's great edging play.