Determine how the flagstone needs to be cut. Take measurements from the area where the flagstone will be placed. Trace the measurements on the top of the flagstone with a marker.
Fit a circular saw with a masonry blade. The blade should have diamond chips imbedded on the edge.
Protect yourself from flying pieces of rock or debris with safety goggles. Wear earplugs to protect your ears from the loud noises associated with cutting rock.
Set up two sawhorses. Place the flagstone on top of the sawhorses.
Cut the flagstone with the circular saw. Cut the flagstone as many times as necessary to shape it to the desired size. The circular saw will not cut thicker stones all the way through. Finish shaping the stone by setting it on a ledge and tapping the cut-away portion with a hammer. The action should finish breaking the stone to size.