Breaking stone the way you want it is not always easy. To increase your chances of doing it properly you need a chisel and hammer. The angle at which you position the chisel and the force you use with the hammer can be the key to breaking the stone properly.
Put on your safety goggles. Place the tip of the chisel at the point on the stone where you want to make the break. Lean the chisel slightly towards the larger mass of the stone. Don't hold it perpendicular to the stone. This will cause the stone to chip rather than break.
Cut a line across the stone where you want it to break. With the chisel positioned at an angle, tap the end of the chisel with the hammer. Continue across the stone to make the line.
Place the chisel point in the middle of the line. Remember to position it at an angle. Use the rock hammer to forcefully hit the top of the chisel. You need to use a heavy hit rather than tap. A tap will just chip the stone rather than break it.
The stone should break apart where you cut the line.