Do not hold onto the bushing chisel. Hold the pneumatic tool perpendicular to the stone so the chisel moves freely, bouncing and spinning over the surface in a random pattern (called "dancing").

The 4-point chisel (above left) is ideal for roughing out your work and will leave a course texture. The 9-point chisel (center above) removes less material but leaves a finer surface. The cup chisel (above right) will not dig and is excellent for curved areas; use it to achieve a smoother texture prior to final finishing.