Set the piece of flagstone that will be split onto a piece of wood. This will provide a strong surface to cut the stone without the danger of causing damage to any structure below the stone.
Draw a line where you wish to cut the stone with a permanent marker. Make the line jagged to preserve the naturally raw characteristics of the stone.
Score the line approximately 1/8 inch deep with a chisel and a small hammer. This will provide you with the size required for the job.
Place the edge of the chisel and split the flagstone with a single hard blow from a sledgehammer.