Stand the bat on its handle. Make sure it is steady.
Pry the flat-head screwdriver under the cap on top of the bat's barrel. It may take some muscle, but it will pop off. There will be a large popping sound as the pressurized barrel's air escapes.
Push tennis balls down into the barrel. Depending on the size of the bat, three of four tennis balls will fit inside the barrel.
Place the cap back on the bat and push it in until it is secure.