Measure the space the concrete stepping stone needs to fit into, and make a small mark with a graphite marker on either edge of the stepping stone to indicate where it needs to be cut. Extend the cutting line across the surface of the stone by connecting the marks using a chalk line or masonry graphite marker.
Set the concrete stepping stone on a flat, sturdy surface. Put on safety glasses and a dust mask.
Attach a masonry blade or carborundum blade to a circular saw and set the cutting depth to 1/2 inch.
Cut into the stone, beginning at one edge and following the cutting line using the circular saw. Push the saw through the stone slowly to prevent overheating.
Adjust the cutting depth on the circular saw to approximately 1 inch and repeat Step 4 to make another, deeper pass over the same cut.
Repeat Steps 4 and 5 as necessary, setting the blade to incrementally deeper settings to complete the cut through the stepping stone.