First of all a little premise on what ballbusting is.
Ballbusting is one of the common practices among CBT (Cock and Ball Torture).
In ballbusting the domme hits, squeezes or punches the balls and cock of the sub either with her foot, knee, hand or whatever else she wants to use. I particularly like to use kitchen tools made of wood (like big spoons) or the wood paddle.
It is important to understand that there are no techniques so to the lady who asked me to explain how to do it I would suggest to attend workshops or demos at events or test on a sub taking it very slowly.
A general advice is to start softly and progressively increase the intensity of the hits keeping in mind that if the hits are too hard the damages can be serious in short and long term.
Short term damages: edemas, bruises, small cuts
Long term damages: swelling (such as hematoma), testicle rupture (this case requires IMMEDIATE medical intervention)
Both short term and long term damages should be avoided so, unless you want to cause the damage on purpose (and this happens often in S/m), learn the techniques with a more experienced domme or at workshops/demos or experiment with your sub taking it very very slowly.
Below, few pictures from the web and two videos.