Lay a piece of plywood on the ground away from driveways and walkways, to prevent damage to the underlying surface.
Lay the flagstone that you want to cut on the plywood and draw a line on it with a pencil to indicate where it must be cut. If needed, lay the flagstone into the desired installation location and draw the line on it there for more precision.
Put on safety glasses and ear plugs. Install a diamond-tipped blade into a masonry saw and plug the saw into an electrical outlet.
Pull the trigger on the masonry saw to start the blade spinning.
Lower the blade toward the flagstone and allow the saw to cut through the stone. Do not push down on the blade. Instead, allow the blade to cut its way naturally through the stone.
Release the trigger on the masonry saw and wait until the blade stops spinning. Set the saw down and remove your ear plugs and safety glasses.