I just do this with my scraps and trimmings, but you could do this with a whole batch if you prefer. You grate up the soap you want to make soap balls from, either by hand with a cheese grater or you can use an electric device used for making salads (in the U.S. there's something called a "Salad Shooter"). Put your grated soap in a bowl, with some room to spare and sprinkle with just enough water to help you mix it all together and have the shavings stick when you press them into a ball. If your soap is fresh, just wetting your hands should be enough. I mix with my fingers until all the shavings are slightly moistened and then pick some up and start squeezing it with my hands and turning it over and over until it's as round as I can get it. Slightly moistening my palms at the end and smoothing the ball will give it a nice finish. Then I set them on racks or on a table top to dry and turn them in a day or two to expose the down side to more air. If you want them to all be the same size, you might want to measure your shavings with something while they are still loose and moistened and after squeezing they should be fairly uniform (a plastic ice-cream scoop might work well for this).