You take the parts to a machine shop that bolts them level to a mill, and they shave off .0xx" which decreases the volume of the combustion chamber to raise the compression. It also brings the valves closer to the pistons which there's very little room for. I wouldn't dare take more than .018" off. You're better off replacing your pistons and doing it the right way because changing the deck height of the head also changes your engine's geometry.